This is a fix for https://github.com/micropython/micropython/issues/2209:
by default a file created using open() uses text translation mode so writing
\n to it will result in the file having \r\n. This is obviously problematic
for binary .mpy files, so provide functions for setting the open mode
and use binary mode in mpy-cross' main().
Building in 32-bit mode was only to reduce binary size on 64-bit machines
and is otherwise not needed. Having it forced to 32-bit meant an
unnecessary dependency on 32-bit libraries that is now removed.
