Fix FAST_MALLOC_GRANS and malloc_many for MANUAL_VDB

Fix FAST_MALLOC_GRANS and malloc_many for MANUAL_VDB
(fix of commit 73d30d2)

This commit actually disables multiple objects allocation in
GC_generic_malloc_many (if MANUAL_VDB and the incremental mode is on);
the proper implementation would require GC_dirty call for each of the
allocated object (except for the last one).

* include/gc_inline.h (GC_FAST_MALLOC_GRANS): Call
GC_end_stubborn_change(my_fl) after GC_generic_malloc_many().
* mallocx.c [MANUAL_VDB] (GC_generic_malloc_many): Always fall back to
GC_generic_malloc (unless !GC_incremental).