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.