Remove hb_large_block field (use 1 extra bit of hb_flags instead)

* allchblk.c (setup_header): Use (hb_flags & LARGE_BLOCK) != 0
instead of hb_large_block.
* private/gc_pmark.h (PUSH_CONTENTS_HDR): Likewise.
* mallocx.c (GC_realloc): Likewise.
* include/private/gc_priv.h (struct hblkhdr): Remove hb_large_block;
define LARGE_BLOCK to access corresponding bit in hb_flags (only if
MARK_BIT_PER_GRANULE).