py/obj: Add MP_NOINLINE to mp_obj_malloc_helper.

As the comment in py/obj.h says:

> Implementing this as a call rather than inline saves 8 bytes per usage.

So in order to get this savings, we need to tell the compiler to never
inline the function.

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

py/obj.c

@@ -38,7 +38,7 @@
 #include "py/stream.h" // for mp_obj_print
 
 // Allocates an object and also sets type, for mp_obj_malloc{,_var} macros.
-void *mp_obj_malloc_helper(size_t num_bytes, const mp_obj_type_t *type) {
+MP_NOINLINE void *mp_obj_malloc_helper(size_t num_bytes, const mp_obj_type_t *type) {
     mp_obj_base_t *base = (mp_obj_base_t *)m_malloc(num_bytes);
     base->type = type;
     return base;