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

[Owen Anderson via llvm-commits]

resistor wrote:

> Can someone confirm whether this is suitable for CHERIoT, the potential user of this patch?

I'm the maintainer of the CHERIoT LLVM downstream.

CHERIoT is not currently using vendor-namespaced relocations. Lack of support for them in LLD is the last blocker before I can start porting CHERIoT to use them.

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