Support atomic_load in rcgen

See e.g. https://gitlab.mpi-sws.org/iris/refinedc/-/blob/0eef2b4be192c9c078c9a9f1228945610a217014/theories/examples/latch/latch.c#L11

added feature frontend labels

closed via commit 289018cd

The modified example works, but I do not want to push on your branch since I am not sure where you want to go with it @msammler. Also, I am not sure if the current solution is general enough since the support of atomic types is not complete (the lvalue case is not yet handled).

Thanks! I will integrate it into the branch when I continue working on it. We can worry about the generality once we have more use cases.