s390/nmi: print machine check interruption code before stopping system

In case a system will be stopped because of e.g. missing validity bits
print the machine check interruption code before the system is stopped.

This is helpful, since up to now no message was printed in such a
case. Only a disabled wait PSW was loaded, which doesn't give a hint of
what went wrong.
Improve this by printing a message with debug information.

Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>