Implement software access and dirty bit management for arm64.

Previously the arm64 pmap did no reference or modification tracking;
all mappings were treated as referenced and all read-write mappings
were treated as dirty. This change implements software management
of these attributes.

Dirty bit management is implemented to emulate ARMv8.1's optional
hardware dirty bit modifier management, following a suggestion from alc.
In particular, a mapping with ATTR_SW_DBM set is logically writeable and
is dirty if the ATTR_AP_RW_BIT bit is clear. Mappings with
ATTR_AP_RW_BIT set are write-protected, and a write access will trigger
a permission fault. pmap_fault() handles permission faults for such
mappings and marks the page dirty by clearing ATTR_AP_RW_BIT, thus
mapping the page read-write.

Reviewed by: alc
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20907