PG_AVAIL1 is available for software use only in the regular x86 page tables.

PG_AVAIL2 and PG_AVAIL3 on the other hand are available for software use
in both regular and nested page tables.

Redefine PG_W as PG_AVAIL3 instead of PG_AVAIL1 so we can use the same macro
regardless of the underlying pmap_type.