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

[Sam Elliott via llvm-commits]

lenary wrote:

@MaskRay I've tagged you in the review because this hits target-independent MC code. We weren't sure of a better way to accomplish what is needed for the RISC-V R_RISCV_VENDOR relocations. We didn't want a separate fixup (as for Relaxation) because we didn't want to emit the R_RISCV_VENDOR relocation if the fixup could be evaluated/applied by the compiler, and that kind of "evaluating one fixup deletes another fixup" seemed complex and error-prone. Hopefully you find our approach reasonable.

There is linker support for these relocations, in https://github.com/qualcomm/eld - which requires the `R_RISCV_VENDOR` relocations - but it does look like the tests are missing from this repo.


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