On amd64, make proc0 pmap initialization slightly more correct. In
particular, switch to the proc0 pmap to have expected %cr3 and PCID
for the thread0 during initialization, and the up to date pm_active
mask.

pmap_pinit0() should be done after proc0->p_vmspace is assigned so
that the amd64 pmap_activate() find the correct curproc pmap.

Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks