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

[James Y Knight via llvm-commits]

jyknight wrote:

Perhaps we could change the generic fence-insertion interface to enable this in the generic interface, but I'm OK with just doing this for now.

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?

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