amd64: store pcids pmap data in pcpu zone

This change eliminates the struct pmap_pcid array embedded into struct
pmap and sized by MAXCPU, which would bloat with MAXCPU increase. Also
it removes false sharing of cache lines, since the array elements are
mostly locally accessed by corresponding CPUs.

Suggested by: mjg
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D39890