ARM_NEW_PMAP

[Ian Lepore]

With commit r280712, we now have a new alternate implementation of the
arm pmap code available (the low-level part of virtual memory handling).
Right now it's disabled by default; if you'd like to help test it, add
to your kernel config:

  options ARM_NEW_PMAP

This new implementation is a complete rewrite, but is modeled after the
mature and well-tested i386 pmap code.  In the testing I've done over
the past few days I've noticed some performance improvement (a 12-minute
kernel build on a wandboard quad is now 10 minutes).  Even more than
performance, though, this new code should bring us better stability.

My testing showed that this does fix the "no l2 bucket..." panic that
showed up recently.

If you need to check whether the new or old code is running on a system,
use "sysctl vm.pmap.pte1.promotions", if that gives an "unknown oid"
error you're running the old code.

Big Thanks go to Svata Kraus and Michal Meloun who contributed this code
(along with an embarrassed apology that it took 2 years to get it
committed).

-- Ian