s390/cmpxchg: remove digits from input constraints

Instead of using a digit for input constraints simply initialize the
corresponding output operand in C code and use a "+" constraint
modifier.

Link: https://lore.kernel.org/r/Y2J8H82B6JhJhrp2@osiris


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>