Determine whether the MMU hardware is capable of updating a page table
entry's access flag and dirty state, and enable this feature when it's
available.

Ensure that we don't overlook a dirty state update that is concurrent
with a call to pmap_enter(). (Previously, all dirty state updates would
have occurred with the containing pmap's lock held, so a page table entry's
dirty state could not have changed while pmap_enter() held that same lock.)

Reviewed by: andrew, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22907