py/py.mk: Use additional CFLAGS to compile string0.c.
py/py.mk: Use additional CFLAGS to compile string0.c.

Otherwise functions like memset might get optimised to call themselves (eg
with gcc 10).  And provide CFLAGS_BUILTIN so these options can be changed
by a port if needed.

Fixes issue #6053.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython