m68knommu: start dump from exception stack

In die_if_kernel() start the stack dump at the exception-time SP, not at the
SP with all the saved registers; the stack below exception-time sp contains
only exception-saved values and is already printed in details just before.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>