Make GC_generic_malloc_ignore_off_page() public

Make GC_generic_malloc_ignore_off_page() public

* include/gc_mark.h (GC_generic_malloc): Refine comment.
* include/gc_mark.h (GC_generic_malloc_ignore_off_page): Move
prototype from gc_priv.h and make it public; add GC_ATTR_MALLOC
attribute.
* include/private/gc_priv.h (GC_generic_malloc): Move comment to
gc_mark.h.
* include/private/gc_priv.h (GC_generic_malloc_ignore_off_page):
Remove GC-inner prototype.
* include/private/gc_priv.h (GC_generic_malloc_inner): Update comment.
* mallocx.c (GC_generic_malloc_ignore_off_page): Likewise.
* mallocx.c (GC_generic_malloc_ignore_off_page): Make public.