add pmap_active_cpus()

For amd64, i386, arm, and riscv, i.e. all architectures except arm64,
the custom implementation is provided since we maintain the bitmask of
active CPUs anyway.

Arm64 uses somewhat naive iteration over CPUs and match current vmspace'
pmap with the argument. It is not guaranteed that vmspace->pmap is the
same as the active pmap, but the inaccuracy should be toleratable.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32360