i386 PAE: avoid atomic for pte_store() where possible.

Instead carefully write upper word, and only than the lower word with
PG_V, for previously invalid ptes. It provides some measurable system
time saving on buildworld.

Reviewed by: markj
Tested by: pho
Measured by: bde (early version)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D19226