s390/atomic_ops: Make use of flag output constraint

With gcc 14.1.0 support for flag output constraint was added for
s390. Use this for __atomic_cmpxchg_bool(). This allows for slightly
better code, since the compiler can generate code depending on the
condition code which is the result of an inline assembly.

The size of the kernel image is reduced by ~12kb.

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>