Enable RISC-V atomic compare and swap

[fintelia]

Now that LLVM has support for atomic CAS on RISC-V it would be good to enable it in rustc for that backend. I think this should just be a matter of updating LLVM (or cherry picking the relevant commit? Not entirely sure how that works) and then flipping the switch.

[fintelia]

What is necessary to move this along?

[nikic]

@fintelia There will be an LLVM update soon (as part of the LLVM monorepo migration), which will pull in the necessary LLVM changes. Afterwards it can be enabled on the rustc side.