ARM: add notify_die() support

Kernel debuggers want to be informed of die() events, so that they
can take some action to allow the problem to be inspected.  Provide
the hook in a similar manner to x86.

Note that we currently don't implement the individual trap hooks.

Acked-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>