amd64 pmap: implement per-superpage locks

The current 256-lock sized array is a problem in the following ways:

• it's way too small
• there are 2 locks per cacheline
• it is not NUMA-aware

Solve these issues by introducing per-superpage locks backed by pages
allocated from respective domains.

This significantly reduces contention e.g. during poudriere -j 104.
See the review for results.

Reviewed by: kib
Discussed with: jeff
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21833