[llvm] [RISCV] Add Vendor Reloc and Fallback Names (PR #116974)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

So, for CHERI-RISC-V, we predate the existence of R_RISCV_VENDOR by many years and thus just use the custom reloc encodings directly with no namespacing. Fixing that isn't worth doing because we're on the path to standardisation, which will yield the necessary relocations existing in the standard range. So your ELF_RISCV_NONSTANDARD_RELOC wouldn't help us, but might be the right shape for "proper" vendor relocations. Which may mean that it's our responsibility to deal with downstream if we're not doing things what is now the "proper" way.

https://github.com/llvm/llvm-project/pull/116974