amd64 pmap: conditionalize per-superpage locks on NUMA

Instead of superpages use. The current code employs superpage-wide locking
regardless and the better locking granularity is welcome with NUMA enabled
even when superpage support is not used.

Requested by: alc
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21982