This follows on from a5324a10747dfba921bb363ed904f05ebb6de010 and allows
mpy-cross to dynamically select whether ARMv7-M instructions are supported
in @micropython.asm_thumb functions.
The config option MICROPY_EMIT_INLINE_THUMB_ARMV7M is no longer needed, it
is now controlled by MICROPY_EMIT_THUMB_ARMV7M.
Signed-off-by: Damien George <damien@micropython.org>