py/mkrules: Automatically build mpy-cross if it doesn't exist.

Commit 41739506589ec8397613c86d8f682fb7f86c0a9f removed automatic building
of mpy-cross, which rebuilt it whenever any of its dependent source files
changed.

But needing to build mpy-cross, and not knowing how, is a frequent issue.
This commit aims to help by automatically building mpy-cross only if it
doesn't exist.  For Makefiles it uses an order-only prerequisite, while
for CMake it uses a custom command.

If MICROPY_MPYCROSS (which is what makemanifest.py uses to locate the
mpy-cross executable) is defined in the environment then automatic build
will not be attempted, allowing a way to prevent this auto-build if needed.

Thanks to Trammell Hudson aka @osresearch for the original idea; see #5760.

Signed-off-by: Damien George <damien@micropython.org>

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