Add pmap_change_prot on arm64

Support changing the protection of preloaded kernel modules by
implementing pmap_change_prot on arm64 and calling it from
preload_protect.

Reviewed by: alc (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32026

(cherry picked from commit a85ce4ad7272ffa4b4649b0ed463341b743e815f)