py/py.mk: Allow to override GC module name.
py/py.mk: Allow to override GC module name.

E.g. one can make an laternative GC implementation in gc_my.c, and use it
with:

    make GC_IMPL=gc_my

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython