[POWERPC] kdump shutdown hook support

This adds hooks into the default_machine_crash_shutdown so drivers can
register a function to be run in the first kernel before we hand off
to the second kernel.  This should only be used in exceptional
circumstances, like where the device can't be reset in the second
kernel alone (as is the case with eHEA).  To emphasize this, the
number of handles allowed to be registered is currently #def to 1.

This uses the setjmp/longjmp code around the call out to the
registered hooks, so any bogus exceptions we encounter will hopefully
be recoverable.

Tested with bogus data and instruction exceptions.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>