[llvm] RISCV, LoongArch: Encode RELAX relocation implicitly (PR #140494)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

This is looking much better, and easier to extend than the previous structure, thanks.

I think there's still a bunch of complexity around forcing relocations in relax mode, for instance in the top of `RISCVAsmParser::parseInstruction` and equivalently in `RISCVTargetELFStreamer::RISCVTargetELFStreamer`, but maybe the work to mark a specific fragment needing relaxation can help simplify this.

https://github.com/llvm/llvm-project/pull/140494