amd64: rework PCPU allocation

Move pcpu KVA out of .bss into dynamically allocated VA at
pmap_bootstrap(). This avoids demoting superpage mapping .data/.bss.
Also it makes possible to use pmap_qenter() for installation of
domain-local pcpu page on NUMA configs.

Refactor pcpu and IST initialization by moving it to helper functions.

Reviewed by: markj
Tested by: pho
Discussed with: jeff
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D21320