[lld] [lld] Add infrastructure for handling RISCV vendor-specific relocations. (PR #159987)

[Owen Anderson via llvm-commits]

resistor wrote:

> It sounds like you want a separate method that's only implemented on RISC-V that takes the vendor as an argument, perhaps? This definitely isn't the right interface.

I'm fine with doing this or @lenary 's suggestion on type erasure. I suppose another option would be to make `getRelExpr` varargs, but that seems undesirable to me.

Do folks feel like a type-erased `arch_specific` argument to `getRelExpr` would be generally useful?

https://github.com/llvm/llvm-project/pull/159987