x86, NMI: NMI-selftest should handle the UP case properly

If no remote cpus are online, then just quietly skip the remote
IPI test for now.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: andi@firstfloor.org
Cc: torvalds@linux-foundation.org
Cc: peterz@infradead.org
Cc: robert.richter@amd.com
Link: http://lkml.kernel.org/r/20111206180859.GR1669@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>