[S390] Fix struct _lowcore layout.

Offsets and sizes are wrong for 32 bit.
Got broken with 866ba284 "[S390] cleanup lowcore.h".

Reported-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>