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Building mpy-cross: this patch adds .exe to the PROG name when building
executables for host (eg mpy-cross) on Windows.  make clean now removes
mpy-cross.exe under Windows.

Building MicroPython: this patch sets MPY_CROSS to mpy-cross.exe or
mpy-cross so they can coexist and use cygwin or WSL without rebuilding
mpy-cross.  The dependency in the mpy rule now uses mpy-cross.exe for
Windows and mpy-cross for Linux.