py: Implement support for "except Exception as var" clause.

For this, needed to implement DELETE_NAME bytecode (because var bound
in except clause is automatically deleted at its end).
http://docs.python.org/3/reference/compound_stmts.html#except :
"When an exception has been assigned using as target, it is cleared at
the end of the except clause."

Based on issues raised during recent review and inconsistency of different
implementations.

Late USB enumeration could clear settings after they had been set.
Now fixed by not clearing some settings on init.

RX was blocking if received characters were not being processed, so
CTRL-C would not be picked up.  Now "fixed" by not blocking, but
instead discarding incoming characters if they overflow the buffer.

py/builtinmath.c: use tgamma() instead of gamma().

Fix OS X detection.

gamma() is now deprecated.

Switch to checking for the __APPLE__ and __MACH__ macros.

Implement __aeabi_f2d and __aeabi_d2f

objint_mpz: Quick&dirty implementation of bitwise operations.

Made solely to unbreak int-long.py test which in turn uncovered thinko
with implementation of inplace ops. On mpz level, bitwise ops implemented
only for same-sign numbers, and are not efficient (unconditional calling of
mpn_cmp() is apparently superfluous).

This was a long-standing regression from converting script from sh to python.

As part of this, rejig the way TIM3 is initialised, since it's now
shared by USB CDC and the blue LED PWM.

Fixed some math functions and added more exceptions.

Fixed some math functions and added more exceptions.