[S390] Add oops_enter()/oops_exit() calls to die().

This is mainly to switch off all potentially debugging stuff that
won't report anything useful after an oops happened.
Besided that setting pause_on_oops will work too, but doesn't make
too much sense on s390.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>