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>
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>
[kyan: trivial merge conflict resolution & compilation fixes]
Signed-off-by: Kyle Yan <kyan@codeaurora.org>
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>