s390/traps: get rid of magic cast for program interruption code

Add a proper union in lowcore to reflect architecture and get rid of a
"magic" cast in order to read the full program interruption code.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>