ARM64: smp: implement arch_trigger_all_cpus_backtrace using IPI

Since ARM64 doesn't have an NMI, send an IPI to all other CPUs
(current cpu prints the stack directly) to capture a backtrace.

Change-Id: Ib90494123205b3bbaa0b244ccde6c7e40a560199
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
[abhimany: fixup the nmi header to account for arm64]
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>