Group all compact fields of GC_arrays to fit in single page

Group all compact fields of GC_arrays to fit in single page

* include/private/gc_priv.h (struct hblkhdr): Reorder hb_inv_sz field
to reduce alignment gaps on 64-bit targets.
* include/private/gc_priv.h (struct _GC_arrays): Reorder
_composite_in_use, _atomic_in_use, _unmapped_bytes, _all_nils,
_trace_addr, _modws_valid_offsets, _root_index, _last_stack fields
so that to have all compact (small-sized) fields in one memory page.