x86: Enable NMI on all cpus on UV

Enable NMI on all cpus in UV system and add an NMI handler
to dump_stack on each cpu.

By default on x86 all the cpus except the boot cpu have NMI
masked off.  This patch enables NMI on all cpus in UV system
and adds an NMI handler to dump_stack on each cpu.  This
way if a system hangs we can NMI the machine and get a
backtrace from all the cpus.

Version 2: Use x86_platform driver mechanism for nmi init, per
           Ingo's suggestion.

Version 3: Clean up Ingo's nits.

Signed-off-by: Russ Anderson <rja@sgi.com>
LKML-Reference: <20100226164912.GA24439@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>