[lld] [llvm] [RISCV] Support RISCV Atomics ABI attributes (PR #84597)

[Alex Bradbury via llvm-commits]

https://github.com/asb commented:

One question in line about the observed output when combining a file without the ABI attribute with one that has it set to 'unknown', and two minor nits. If we resolve that question, it all looks good to me - thanks for your patience!

https://github.com/llvm/llvm-project/pull/84597