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

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Is the expectation that downstream tools support `R_RISCV_CUSTOM<n>` as non-canonical aliases when they are given a defined meaning? There doesn't seem to be an ELF_RELOC_ALIAS equivalent to allow for that, so such downstream (ourselves included) would have to add extra cases to RISCVAsmBackend::getFixupKind manually. We can do that if needed, but it'd be nicer if upstream supported this more directly.

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