unix/alloc: Use MP_REGISTER_ROOT_POINTER() for mmap_region_head.

This uses MP_REGISTER_ROOT_POINTER() to register mmap_region_head and
removes the same from mpconfigport.h.

Signed-off-by: David Lechner <david@pybricks.com>

ports/unix/alloc.c

@@ -104,4 +104,6 @@ void ffi_closure_free(void *ptr) {
 }
 #endif
 
+MP_REGISTER_ROOT_POINTER(void *mmap_region_head);
+
 #endif // MICROPY_EMIT_NATIVE || (MICROPY_PY_FFI && MICROPY_FORCE_PLAT_ALLOC_EXEC)

ports/unix/mpconfigport.h

@@ -305,7 +305,6 @@ struct _mp_bluetooth_nimble_malloc_t;
 #endif
 
 #define MICROPY_PORT_ROOT_POINTERS \
-    void *mmap_region_head; \
     MICROPY_BLUETOOTH_ROOT_POINTERS \
 
 // We need to provide a declaration/definition of alloca()