s390/cmpxchg: Use arch_cmpxchg() instead of __atomic_cmpxchg()

Use arch_cmpxchg() instead of __atomic_cmpxchg() for the
arch_atomic_cmpxchg() implementations. arch_cmpxchg() generates
the same code and doesn't need a cast like it is required for
arch_atomic64_cmpxchg().

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