x86, NMI: Add NMI IPI selftest

The previous patch modified the stop cpus path to use NMI
instead of IRQ as the way to communicate to the other cpus to
shutdown.  There were some concerns that various machines may
have problems with using an NMI IPI.

This patch creates a selftest to check if NMI is working at
boot. The idea is to help catch any issues before the machine
panics and we learn the hard way.

Loosely based on the locking-selftest.c file, this separate file
runs a couple of simple tests and reports the results.  The
output looks like:

...
Brought up 4 CPUs
----------------
| NMI testsuite:
--------------------
  remote IPI:  ok  |
   local IPI:  ok  |
--------------------
Good, all   2 testcases passed! |
---------------------------------
Total of 4 processors activated (21330.61 BogoMIPS).
...

Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Richter <robert.richter@amd.com>
Cc: seiji.aguchi@hds.com
Cc: vgoyal@redhat.com
Cc: mjg@redhat.com
Cc: tony.luck@intel.com
Cc: gong.chen@intel.com
Cc: satoru.moriya@hds.com
Cc: avi@redhat.com
Cc: Andi Kleen <andi@firstfloor.org>
Link: http://lkml.kernel.org/r/1318533267-18880-3-git-send-email-dzickus@redhat.com


Signed-off-by: Ingo Molnar <mingo@elte.hu>