amd64: Initialize kernel_pmap's active CPU set to all_cpus

This is in preference to simply filling the cpuset, and allows the
conditional in pmap_invalidate_cpu_mask() to be elided.

Also export pmap_invalidate_cpu_mask() outside of pmap.c for use in a
subsequent commit.

Suggested by: kib
Reviewed by: alc, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 71e6e9da225aede95f6813a0fcf886538d0da9fe)