s390/atomic: refactor atomic primitives

Rework atomic.h to make the low level functions avaible for use
in other headers without using atomic_t, e.g. in bitops.h.

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