Implement wiring changes on superpage mappings. Previously, a superpage
mapping was unconditionally demoted by pmap_unwire(), even if the wiring
change applied to the entire superpage mapping.
Rewrite a comment to use the arm64 names for bits in a page table entry.
Previously, the bits were referred to by their x86 names.
Use atomic_"op"_64() instead of atomic_"op"_long() to update a page table
entry in order to match the prevailing style in this file.
MFC after: 10 days
dd738ba879