[llvm] [RISCV] Add additional fence for amocas when required by recent ABI change (PR #101023)

[Alex Bradbury via llvm-commits]

asb wrote:

> This does seem unfortunate for performance, though -- I hope someone's planning to implement the proper "A7" atomic abi mappings in the Clang 20 timeframe, so that these extraneous fences can go away?

I agree it would be nice to have this, even if rolling out the A7 mappings in standard Linux distros is hard to envisage due to ABI compatibility. All the infrastructure in terms of ELF ABI targets etc is there.

Ping for anyone to give a LGTM so I can get this merged. Many thanks.

https://github.com/llvm/llvm-project/pull/101023