Fix data race when getting object size in explicitly-typed allocators

Fix data race when getting object size in explicitly-typed allocators
(Cherry-pick commit eb33bda5 from 'release-7_6' branch.)

* typd_mlc.c (COMPLEX): Reformat comment.
* typd_mlc.c (GC_malloc_explicitly_typed,
GC_malloc_explicitly_typed_ignore_off_page, GC_calloc_explicitly_typed):
Always use BYTES_TO_GRANULES(GC_size(op)) instead of GC_size_map[lb] to
determine size of the allocated object in granules (because the value
of GC_size_map[lb] might be updated by another thread since the value
use in GC_malloc_kind); add comment.