Now that some ports support multiple architectures (eg esp32 has both
Xtensa and RISC-V CPUs) it's no longer possible to set mpy-cross flags
based on the target, eg `./run-tests.py --target esp32`. Instead this
commit makes it so the `-march=xxx` argument to mpy-cross is detected
automatically via evaluation of `sys.implementation._mpy`.
Signed-off-by: Damien George <damien@micropython.org>
