powerpc/kdump: Use setjmp/longjmp to handle kdump and system reset recursion

We can handle recursion caused by system reset by reusing the crash
shutdown fault handler.

Since we don't have an OS triggerable NMI, if all CPUs don't make it
into kdump then we tell the user to issue a system reset. However if
we have a panic timeout set we cannot wait forever and must continue
the kdump.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>