s390: remove diag 44 calls from cpu_relax()

Simplify cpu_relax() to a simple barrier(). Performance wise this doesn't
seem to make any big difference anymore, since nearly all lock variants
have directed yield semantics in the meantime.
Also this makes s390 behave like all other architectures.

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