Fix concurrent stacks for more realistic CAS

https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/165 made CAS more realistic, which broke the concurrent stacks in the ci/gen_proofmode branch. All four of them contain a CAS comparing arbitrary values, which I do not think is possible in a lock-free way.

I asked Danny to have a look at this; I don't think he has an account here.