s390/uaccess: use symbolic names for inline assembly operands

Improve readability of the uaccess inline assemblies by using symbolic
names for all input and output operands.

Except for debug sections the output of "objdump -Dr" of the uaccess object
file is identical before/after this change.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>