[llvm] [RISCV] Add support for vendor relocations on Xqci extensions (PR #135400)

[Sam Elliott via llvm-commits]

lenary wrote:

@tclin914 I've tagged you because you'll likely want this functionality for the andes extension with relocations, and it would be good to avoid duplicating this work.

https://github.com/llvm/llvm-project/pull/135400