[sgen] Comment out __GNUC__ version of OBJ_BITMAP_FOREACH_PTR (bxc#14834)

The __GNUC__ version of OBJ_BITMAP_FOREACH causes a SIGSEGV, so it is
better to fallback to the other version until a fix is found.

Closes https://bugzilla.xamarin.com/show_bug.cgi?id=14834