[llvm] [GitHub][RISCV] Restrict backend:RISC-V labelling to Clang and LLVM (PR #192429)

[Luke Lau via llvm-commits]

lukel97 wrote:

> My feeling is seeing LLD and LLDB PRs is useful to me.
> 
> I don't like the libc PRs which just change the lists of entry points. Those are super noisy.
> 
> I'm on the fence about seeing changes to VPlan/SLP (usually via test paths) but I realise it is important to others :)

I'm happy to keep LLD and LLDB if others find them useful. I also agree for VPlan/SLP we already have the vectorizers label so I don't think we need to specifically add the backend tags for it. 

This might be better to discuss at the next RISC-V sync up to check what we want included

https://github.com/llvm/llvm-project/pull/192429