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

[Sam Elliott via llvm-commits]

lenary 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 :) 

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