Unify amd64 and i386 vmspace0 pmap activation.

Add pmap_activate_boot() for i386, move the invocation on APs from MD
init_secondary() to x86 init_secondary_tail().

Suggested by: alc
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
Approved by: re (marius)
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D16893