s390/atomic_ops: Improve __atomic_set() for small values

Use mvhi/mvghi for small constant values within the __atomic_set()
inline assemblies. This avoids loading the specified value into a
register.

The size of the kernel image is reduced by ~1.2kb.

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