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
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32792