No std-int types are used in gc.h, but size_t is which needs stddef.h. Signed-off-by: Damien George <damien@micropython.org>
The native code no longer holds live GC pointers so doesn't need to be scanned.
This is to keep the top-level directory clean, to make it clear what is core and what is a port, and to allow the repository to grow with new ports in a sustainable way.