s390/head: replace hard coded values with constants

A couple of the low core offsets are repeatedly used throughout head.S
and are hard coded. Replace them with corresponding constants from
asm-offsets.h to improve readability.

Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>