Fix GC_finalized_malloc failure on disclaim_test

Fix GC_finalized_malloc failure on disclaim_test
(revert part of commit 63fd11d)

The finalizer closure is placed in the first word in order to use the
lower bits to distinguish live objects from objects on the free list.
The downside of this is that we need one-word offset interior pointers,
and that GC_base does not return the start of the user region.

* fnlz_mlc.c (GC_finalized_disclaim): Move finalizer closure for
finalized object kinds back to the start of objects.
* fnlz_mlc.c (GC_init_finalized_malloc): Call
GC_register_displacement_inner() as GC_finalized_malloc returns
displaced pointer.
* fnlz_mlc.c (GC_core_finalized_malloc, GC_finalized_malloc): Store
fclos in first word of created object (instead of the last word);
return pointer to object displaced by 1 word.