arm64 pmap: Add ATTR_CONTIGUOUS support [Part 3]

Introduce L3C promotion of base page mappings. When the base page size
is 4KB, use ATTR_CONTIGUOUS to promote 16 aligned, contiguous base page
mappings to a 64KB mapping. Alternatively, when the base page size is
16KB, use ATTR_CONTIGUOUS to promote 128 aligned, contiguous base page
mappings to a 2MB mapping.

Given the frequency of L3C counter updates, switch to per-CPU counters
to avoid cache line ping ponging.

Revise the L3C counter descriptions to reflect the fact that the size
of an L3C mapping varies depending on the base page size.

Co-authored-by: Eliot Solomon <ehs3@rice.edu>
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44983