amd64 pmap: convert to counter(9), add PV and pagetable page counts

This change converts most of the counters in the amd64 pmap from
global atomics to scalable counter(9) counters. Per discussion
with kib@, it also removes the handrolled per-CPU PCID save count
as it isn't considered generally useful.

The bulk of these counters remain guarded by PV_STATS, as it seems
unlikely that they will be useful outside of very specific debugging
scenarios. However, this change does add two new counters that
are available without PV_STATS. pt_page_count and pv_page_count
track the number of active physical-to-virtual list pages and page
table pages, respectively. These will be useful in evaluating
the memory footprint of pmap structures under various workloads,
which will help to guide future changes in this area.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D28923