s390/cmpxchg: Provide arch_cmpxchg128_local()

Just like x86 and arm64 provide a trivial arch_cmpxchg128_local()
implementation by mapping it to arch_cmpxchg128().

Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>