s390/atomic: Implement arch_atomic_inc() / arch_atomic_dec()

s390/atomic: Implement arch_atomic_inc() / arch_atomic_dec()

Implement arch_atomic_inc() / arch_atomic_dec() functions which result
in a single instruction if compiled for z196 or newer architectures.

Reduces the kernel image size by ~6K (defconfig):

bloat-o-meter:
add/remove: 0/0 grow/shrink: 12/1005 up/down: 106/-6404 (-6298)

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>