Hopefully fix a few problems that could cause hangs in SMP mode.

Hopefully fix a few problems that could cause hangs in SMP mode.
1)	Make sure that the region mapped by a 4MB page is
	properly aligned.
2)	Don't turn on the PG_G flag in locore for SMP.  I plan
	to do that later in startup anyway.
3)	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...