ARM: smp: implement arch_trigger_all_cpus_backtrace using IPI

Based on a rough patch by frank.rowand@am.sony.com

Since ARM doesn't have an NMI (fiq's are not always available),
send an IPI to all other CPUs (current cpu prints the stack directly)
to capture a backtrace.

Change-Id: I8b163c8cec05d521b433ae133795865e8a33d4e2
Signed-off-by: Dima Zavin <dima@android.com>