s390/cmpxchg: get rid of gcc atomic builtins

s390 is the only architecture in the kernel which makes use of gcc's
atomic builtin functions. Even though I don't see any technical
problem with that right now, remove this code and open-code
compare-and-swap loops again, like every other architecture is doing
it also.
We can switch to a generic implementation when other architectures are
doing that also.

See also https://lwn.net/Articles/586838/

 for forther details.

This basically reverts commit f318a122 ("s390/cmpxchg: use
compiler builtins").

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