Add assertions to ensure ADD_CALL_CHAIN is called holding the lock

Add assertions to ensure ADD_CALL_CHAIN is called holding the lock
(code refactoring)

* alloc.c (GC_collect_a_little_inner, GC_allocobj): Add assertion that
the allocation lock is held.
* dbg_mlc.c (GC_store_debug_info_inner): Likewise.
* dbg_mlc.c [DBG_HDRS_ALL] (GC_debug_generic_malloc_inner,
GC_debug_generic_malloc_inner_ignore_off_page): Likewise.
* finalize.c [!GC_TOGGLE_REFS_NOT_NEEDED] (ensure_toggleref_capacity):
Likewise.
* specific.c [USE_CUSTOM_SPECIFIC] (GC_key_create_inner): Likewise.
* alloc.c (GC_allocobj): Remove comment (about the lock).
* dbg_mlc.c [DBG_HDRS_ALL] (GC_debug_generic_malloc_inner): Likewise.