[lld] [wip][LLD] Support RISCV vendor-specific relocations. (PR #168497)

[Owen Anderson via llvm-commits]

resistor wrote:

Superseded by https://github.com/llvm/llvm-project/pull/169273 (static relocation support) and https://github.com/llvm/llvm-project/pull/169274 (dynamic relocation support).

https://github.com/llvm/llvm-project/pull/168497