Loading a pointer by indexing into the native function table mp_fun_table,
rather than loading an immediate value (via a PC-relative load), uses less
code space.

Useful for position independent code, and implementing state machines.

For all but the last pass the assembler only needs to count how much space
is needed for the machine code, it doesn't actually need to emit anything.
The dummy_data just uses unnecessary RAM and without it the code is not
any more complex (and code size does not increase for Thumb and Xtensa
archs).

This patch adds the MICROPY_EMIT_INLINE_XTENSA option, which, when
enabled, allows the @micropython.asm_xtensa decorator to be used.

The following opcodes are currently supported (ax is a register, a0-a15):

    ret_n()
    callx0(ax)
    j(label)
    jx(ax)

    beqz(ax, label)
    bnez(ax, label)
    mov(ax, ay)
    movi(ax, imm) # imm can be full 32-bit, uses l32r if needed

    and_(ax, ay, az)
    or_(ax, ay, az)
    xor(ax, ay, az)
    add(ax, ay, az)
    sub(ax, ay, az)
    mull(ax, ay, az)

    l8ui(ax, ay, imm)
    l16ui(ax, ay, imm)
    l32i(ax, ay, imm)
    s8i(ax, ay, imm)
    s16i(ax, ay, imm)
    s32i(ax, ay, imm)
    l16si(ax, ay, imm)
    addi(ax, ay, imm)

    ball(ax, ay, label)
    bany(ax, ay, label)
    bbc(ax, ay, label)
    bbs(ax, ay, label)
    beq(ax, ay, label)
    bge(ax, ay, label)
    bgeu(ax, ay, label)
    blt(ax, ay, label)
    bnall(ax, ay, label)
    bne(ax, ay, label)
    bnone(ax, ay, label)

Upon entry to the assembly function the registers a0, a12, a13, a14 are
pushed to the stack and the stack pointer (a1) decreased by 16.  Upon
exit, these registers and the stack pointer are restored, and ret.n is
executed to return to the caller (caller address is in a0).

Note that the ABI for the Xtensa emitters is non-windowing.