Prior to this patch, native code would use a full nlr_buf_t for each
exception handler (try-except, try-finally, with).  For nested exception
handlers this would use a lot of C stack and be rather inefficient.

This patch changes how exceptions are handled in native code by setting up
only a single nlr_buf_t context for the entire function, and then manages a
state machine (using the PC) to work out which exception handler to run
when an exception is raised by an nlr_jump.  This keeps the C stack usage
at a constant level regardless of the depth of Python exception blocks.

The patch also fixes an existing bug when local variables are written to
within an exception handler, then their value was incorrectly restored if
an exception was raised (since the nlr_jump would restore register values,
back to the point of the nlr_push).

And it also gets nested try-finally+with working with the viper emitter.

Broadly speaking, efficiency of executing native code that doesn't use
any exception blocks is unchanged, and emitted code size is only slightly
increased for such function.  C stack usage of all native functions is
either equal or less than before.  Emitted code size for native functions
that use exception blocks is increased by roughly 10% (due in part to
fixing of above-mentioned bugs).

But, most importantly, this patch allows to implement more Python features
in native code, like unwind jumps and yielding from within nested exception
blocks.

Useful for position independent code, and implementing state machines.

Can be used, for example, to configure external SPI flash using a hardware
SPI interface (code to be put in a board's bdev.c file):

    STATIC const spi_proto_cfg_t hard_spi_bus = {
        .spi = &spi_obj[5],
        .baudrate = 10000000,
        .polarity = 0,
        .phase = 0,
        .bits = 8,
        .firstbit = SPI_FIRSTBIT_MSB,
    };

    STATIC mp_spiflash_cache_t spi_bdev_cache;

    const mp_spiflash_config_t spiflash_config = {
        .bus_kind = MP_SPIFLASH_BUS_SPI,
        .bus.u_spi.cs = pin_A0,
        .bus.u_spi.data = (void*)&hard_spi_bus,
        .bus.u_spi.proto = &spi_proto,
        .cache = &spi_bdev_cache,
    };

    spi_bdev_t spi_bdev;

These POSIX wrappers are assumed to be passed a concrete stream object so
it is more efficient (eg on nan-boxing builds) to pass in the pointer
rather than mp_obj_t, because then the users of these functions only need
to store a void* (and mp_obj_t may be wider than a pointer).  And things
would be further improved if the stream protocol functions eventually took
a pointer as their first argument (instead of an mp_obj_t).

This patch is a step to getting ussl/axtls compiling on nan-boxing builds.

See issue #3085.

mpy-cross is a host, not target binary. It should not be build with the
target compiler, compiler options and other settings. For example,

If someone currently tries to build from pristine checkout the unix port
with the following command:

    make CROSS_COMPILE=arm-linux-gnueabihf-

then mpy-cross will be built with arm-linux-gnueabihf-gcc and of course
won't run on the host, leading to overall build failure.

This situation was worked around for some options in 1d8c3f4c, so add
MICROPY_FORCE_32BIT and CROSS_COMPILE to that set too.

The aim here is to have spi.c contain the low-level SPI driver which is
independent (not fully but close) of MicroPython objects and methods, and
the higher-level bindings are separated out to pyb_spi.c and machine_spi.c.

Among other things, this requires putting bootloader object files in to
their relevant .a archive, so that they can be correctly referenced by the
ESP IDF's linker script.

Otherwise there is the possibility that n_free starts out non-zero from the
previous iteration, which may have found a few (but not enough) free blocks
at the end of the heap.  If this is the case, and if the very first blocks
that are scanned the second time around (starting at
gc_last_free_atb_index) are found to give enough memory (including the
blocks at the end of the heap from the previous iteration that left n_free
non-zero) then memory will be allocated starting before the location that
gc_last_free_atb_index points to, most likely leading to corruption.

This serious bug did not manifest itself in the past because a gc_collect
always resets gc_last_free_atb_index to point to the start of the GC heap,
and the first block there is almost always allocated to a long-lived
object (eg entries from sys.path, or mounted filesystem objects), which
means that n_free would be reset at the start of the search loop.

But with threading enabled with the GIL disabled it is possible to trigger
the bug via the following sequence of events:

1. Thread A runs gc_alloc, fails to find enough memory, and has a non-zero
   n_free at the end of the search.
2. Thread A calls gc_collect and frees a bunch of blocks on the GC heap.
3. Just after gc_collect finishes in thread A, thread B takes gc_mutex and
   does an allocation, moving gc_last_free_atb_index to point to the
   interior of the heap, to a place where there is most likely a run of
   available blocks.
4. Thread A regains gc_mutex and does its second search for free memory,
   starting with a non-zero n_free.  Since it's likely that the first block
   it searches is available it will allocate memory which overlaps with the
   memory before gc_last_free_atb_index.

The default SYSCLK frequency is reduced to 192MHz because SDRAM requires it
to be 200MHz or less.

To align with recent changes to sdram.c.

- Allow configuration by a board of autorefresh number and burst length.
- Increase MPU region size to 8MiB.
- Make SDRAM region cacheable and executable.

If a port would like to expose the configuration of transmit and/or receive
buffers then it can use these arguments.

"coverage" build uses different BUILD directory comparing to the normal
build. Previously, any build picked up libaxtls.a from normal build's
directory, but that was fixed recently. So, for each build, we must
build axtls explicitly.

This fixes Travis build in particular.

Use -L$(BUILD), not -Lbuild. Otherwise, builds for different archs/subarchs
using different values of BUILD may fail.

Use overrideable properties instead of hardcoding the use of the
default cl executable used by msvc toolsets. This allows using
arbitrary compiler commands for qstr header generation.
The CLToolExe and CLToolPath properties are used because they are,
even though absent from any official documentation, the de-facto
standard as used by the msvc toolsets themselves.

Without this patch, on 64-bit architectures the "1 << (small_int_bits - 1)"
is computed using only 32-bit values (since small_int_bits is a uint8_t)
and so will overflow (and give the wrong result) if small_int_bits is
larger than 32.

Requesting a baudrate of X should never configure the peripheral to have a
baudrate greater than X because connected hardware may not be able to
handle higher speeds.  This patch makes sure to round the prescaler up so
that the actual baudrate is rounded down.

Input files like basics/string_format.py and float/string_format.py have
the same basename so using that name for writing the output (.exp and .out
files) when both tests fail, results in the output of the first one being
overwritten.

Avoid this by using unique names for the output, replacing path characters
with underscores.

Following standard practice for defining custom exceptions.

Otherwise it can clash with time_t from the C standard include headers.

There is no need to have three copies of the exception object on the top of
the native value stack.  Instead, the values on the stack should be the
first two items in an nlr_buf_t: the prev pointer and the ret_val pointer.
This is all that is needed and is what the rest of the native emitter
expects is on the stack.

This patch is essentially an optimisation.  Behaviour is unchanged,
although the stack layout for native exception handling now makes more
sense.

A native function allocates space on its C stack for mp_code_state_t,
followed by its Python stack, then its locals.  This patch makes sure that
the native function actually starts at the start of its Python stack,
rather than at the start of mp_code_state_t (which didn't lead to any
issues so far because the mp_code_state_t is unused after the native
function sets itself up).

It should work reliably now.

On x86 archs (both 32 and 64 bit) a bool return value only sets the 8-bit
al register, and the higher bits of the ax register have an undefined
value.  When testing the return value of such cases it is required to just
test al for zero/non-zero.  On the other hand, checking for truth or
zero/non-zero on an integer return value requires checking all bits of the
register.  These two cases must be distinguished and handled correctly in
generated native code.  This patch makes sure of this.

For other supported native archs (ARM, Thumb2, Xtensa) there is no such
distinction and this patch does not change anything for them.

The Python documentation recommends to pass the command as a string when
using Popen(..., shell=True).  This is because "sh -c <string>" is used to
execute the command and additional arguments after the command string are
passed to the shell itself (not the executing command).

https://docs.python.org/3.5/library/subprocess.html#subprocess.Popen

And increase sampling time to get better results for internal channels.

Prior to this patch, if VBAT was read via ADC.read() or
ADCAll.read_channel(), then it would remain enabled and subsequent reads
of TEMPSENSOR or VREFINT would not work.  This patch makes sure that VBAT
is disabled for all cases that it could be read.