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
f318a1229bd8 ("s390/cmpxchg: use
compiler builtins").
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>