s390/entry: Add base register to SIEEXIT macro

In preparation of having lowcore at different address than zero,
add the base register to SIEEXIT. No functional change, because
%r0 is passed to the macro.

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>