New 64-bit pmap

[Nathan Whitehorn]

I wrote a new memory management implementation for 64-bit PPC systems 
last week to get greater concurrency on big SMP systems. On a 32-thread 
POWER8 system, this results in a factor of two speedup in buildworld 
time. Smaller systems should see little change.

Since this is a nearly ground-up rewrite of the pmap layer, I'd 
appreciate any testing before pushing the code into HEAD. The patch is 
at http://people.freebsd.org/~nwhitehorn/ppc64-new-pmap.diff.
-Nathan