[S390] Change atomic_read/set to inline functions with barrier semantics.

After doing some tests this seems to be the best variant for s390 and
should be correct as well. With gcc 4.2.1 we get the following kernel
image sizes using the default configuration:

atomic_t type volatile, atomic_read/set defines   5311824 bytes
atomic_t type int, atomic_read/set defines        5270864 bytes
atomic_t type int, atomic_read/set inline asm     5279056 bytes
atomic_t type int, atomic_read/set inline barrier 5270864 bytes

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