[S390] atomic: use ACCESS_ONCE() for atomic_read()

Let's make atomic_read() and atomic_set() behave like on all/most other
architectures. Generated code is identical with gcc 4.5.2.

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