s390/barrier: cleanup barrier functions

s390 really has no eieio instruction, so get rid of the implied ppc
semantics and in addition change mb() into a function.
Also remove SYNC_OTHER_CORES() since it is unused.

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