nmi: create generic NMI backtrace implementation

x86s NMI backtrace implementation (for arch_trigger_all_cpu_backtrace())
is fairly generic in nature - the only architecture specific bits are
the act of raising the NMI to other CPUs, and reporting the status of
the NMI handler.

These are fairly simple to factor out, and produce a generic
implementation which can be shared between ARM and x86.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>