Fix large object base computation in PUSH_CONTENTS() if MARK_BIT_PER_OBJ

Fix large object base computation in PUSH_CONTENTS() if MARK_BIT_PER_OBJ

Issue #177 (bdwgc).

* include/private/gc_pmark.h [MARK_BIT_PER_OBJ] (PUSH_CONTENTS_HDR):
Do not call LONG_MULT() if inv_sz == LARGE_INV_SZ; set base to
hhdr->hb_block if inv_sz == LARGE_INV_SZ regardless of low_prod>>16
value; use EXPECT(FALSE) for inv_sz == LARGE_INV_SZ expression;
remove FIXME about offset; adjust assertion to allow hb_block==current.