s390/entry: Make system_call() ready for lowcore relocation

In preparation of having lowcore at different address than zero,
add the base register to all lowcore accesses in system_call().

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