[llvm] [RISCV][NFC] Refactor Vendor Reloc Declarations (PR #138226)

[Sam Elliott via llvm-commits]

lenary wrote:

> A more general question - is there a reason you call it nonstandard_reloc and not vendor_reloc? Can there be other types of non_standard relocations apart from the vendor ones?

The ABI is reasonably inconsistent about the use of both terms, but the table says "Reserved for nonstandard extensions", and the relevant section was "Nonstandard relocations" until i added "(a.k.a. Vendor-Specific Relocations)" recently.

https://github.com/llvm/llvm-project/pull/138226