s390/entry: Make early program check handler relocated lowcore aware

s390/entry: Make early program check handler relocated lowcore aware

Add the missing pieces so the early program check handler also works
with a relocated lowcore. Right now the result of an early program
check in case of a relocated lowcore would be a program check loop.

Fixes: 8f1e70adb1a3 ("s390/boot: Add cmdline option to relocate lowcore")
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>