ARM64: smp: Save CPU registers before IPI_CPU_STOP processing

When a kernel panic occurs on one CPU, other CPUs are instructed to stop
execution via the IPI_CPU_STOP message. These other CPUs dump their stack,
which may not be good enough to reconstruct their context to perform
post-mortem analysis. Dump each CPU's context (before it started
procesing the IPI) into a globally accessible structure and print them on
the dmesg/console to allow for easier post-mortem debugging.

Change-Id: Ifd7589af4327992540196c87f8b640045d7eaf19
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>