zephyr/main: Use mp_stack API instead of local pointer for stack top.
zephyr/main: Use mp_stack API instead of local pointer for stack top.

The MP_STATE_THREAD(stack_top) is always available so use it instead of
creating a separate variable.  This also allows gc_collect() to be used as
an independent function, without real_main() being called.

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