s390/lib,xor: get rid of register asm

Looking at the generate code this was just a micro-optimization.
However given that as many register asm constructs as possible
will be removed from s390 code, remove this one as well.

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