Do not lose dirty bits for removing PROT_WRITE on arm64.

Arm64 pmap interprets accessed writable ptes as modified, since
ARMv8.0 does not track Dirty Bit Modifier in hardware. If writable bit
is removed, page must be marked as dirty for MI VM.

This change is most important for COW, where fork caused losing
content of the dirty pages which were not yet scanned by pagedaemon.

Reviewed by: alc, andrew
Reported and tested by: Mark Millard <markmi@dsl-only.net>
PR: 217138, 217239
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks