Change GC card making to use object head, bug fix for volatile sput obj

This CL changes the way we mark GC card to consistently use the object
head (previously, we marked somewhere in the object - often the head, but
not always).  Also, previously a coding error caused us to skip the card
mark for OP_APUT_OBJECT_VOLATILES.  Fixed here.

Change-Id: I133ef6395c51a0466c9708209b08e79c3083aff2