Hopefully fix a few problems that could cause hangs in SMP mode.
- Make sure that the region mapped by a 4MB page is
properly aligned.
- Don't turn on the PG_G flag in locore for SMP. I plan
to do that later in startup anyway.
- Make sure the 2nd processor has PSE enabled, so that 4MB
pages don't hose it.
We don't use PG_G yet on SMP -- there is work to be done to make that
work correctly. It isn't that important anyway...