s390/atomic: remove small optimization to fix clang build

With commit f0cbd3b8 ("s390/atomic: circumvent gcc 10 build
regression") there was an attempt to workaroud a gcc build bug,
however with the workaround a similar problem with clang appeared.
It was recommended to use a workaround which would fail again with
gcc. Therefore simply remove the optimization. It is just not worth
the effort.

Besides that all of this will be changed to use compiler atomic
builtins instead anyway.

See https://reviews.llvm.org/D90231
and https://reviews.llvm.org/D91786



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