s390/atomic: various small cleanups

- add a typecheck to the defines to make sure they operate on an atomic_t
- simplify inline assembly constraints
- keep variable names common between functions

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>