[llvm] [RISCV][NFC] Refactor Vendor Reloc Declarations (PR #138226)

[Sam Elliott via llvm-commits]

lenary wrote:

@resistor FYI this might overlap with your work.

I'm planning to merge this sometime this week. I just need to negotiate how our downstream merge process works, as we have some lag and breaking the build will only increase the lag. 

https://github.com/llvm/llvm-project/pull/138226