[gc] add MEMORY_DEBUG support to the GMS gc

fix and enable -DMEMORY_DEBUG for the GMS gc,
add DETAIL_MEMORY_DEBUG lines for all sanity checked PMCs,
and add a single MEMORY_DEBUG line when write_barrier'ing a PMC

we never caught up with the change from the List_Item_Header iterator
to POINTER_ARRAY_ITER.
Fixes GH issue #1073
Merge branch 'rurban/gms_debug-gh1073'