[lld] [llvm] Reapply "[RISCV] Support RISCV Atomics ABI attributes (#84597)" (PR #90266)

[Alex Bradbury via llvm-commits]

asb wrote:

> Ping. Are we happy w/ this as an `-mattr`, or should it be guarded by another kind of flag?

Sorry, I'd missed the update somehow. I'd actually been thinking a cl::opt flag (much like AArch64 has aarch64-mark-bti-property). I'd say the `+abi-attr`/`FeatureAbiAttributes` is confusingly named as well, as it controls only the atomics ABI attributes rather than all of them.

Unless anyone else feels differently I'd think the cl::opt is most consistent with how we usually do this (and easy to pass through from clang with `-mllvm`). The precise name doesn't matter much, as long as it's clear it relates to atomic abi attribute / tags in some way I promise I won't bikeshed beyond that!

https://github.com/llvm/llvm-project/pull/90266