Fix hocap for more realistic CAS

https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/165 made CAS more realistic, which broke HoCAP in the ci/gen_proofmode branch. hocap/fg_bag.v contains a CAS comparing arbitrary values, which I do not think is possible in a lock-free way.

@dfrumin could you have a look at this?