s390/spinlock: Use flag output constraint for arch_cmpxchg_niai8()

Add a new variant of arch_cmpxchg_niai8() which makes use of the flag
output constraint, which allows the compiler to generate slightly better
code. Also rename arch_cmpxchg_niai8() to arch_try_cmpxchg_niai8() which
reflects the purpose of the function and makes it consistent with other
"try" variants.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>