ARM: add basic support for on-demand backtrace of other CPUs

As we now have generic infrastructure to support backtracing of other
CPUs in the system on lockups, we can start to implement this for ARM.
Initially, we add an IPI based implementation, as the GIC code needs
modification to support the generation of FIQ IPIs, and not all ARM
platforms have the ability to raise a FIQ in the non-secure world.

This provides us with a "best efforts" implementation in the absence
of FIQs.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>