Hide and rename GC_dirty to GC_dirty_async[_inner]

Hide and rename GC_dirty to GC_dirty_async[_inner]
(code refactoring)

Also, do not call GC_dirty_async_inner unless GC_incremental.

* include/private/gc_priv.h [MANUAL_VDB] (GC_dirty_async_inner):
Declare GC_INNER function.
* include/private/gc_priv.h (GC_dirty_async): New macro (calls
GC_dirty_async_inner if MANUAL_VDB and GC_incremental.
* mallocx.c [MANUAL_VDB] (GC_dirty): Remove declaration.
* mark.c [MANUAL_VDB && THREADS] (GC_dirty): Likewise.
* mallocx.c (GC_end_stubborn_change): Remove GC_ATTR_UNUSED; call
GC_dirty_async (unconditionally) instead of GC_dirty.
* mark.c [THREADS] (GC_mark_and_push_stack): Call GC_dirty_async
instead of GC_dirty.
* os_dep.c [MANUAL_VDB] (GC_dirty): Rename to GC_dirty_async_inner;
add GC_INNER; change argument type from ptr_t to const void*.