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

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

I second this. If needed, we can create new RISC-V labels for other sub-projects. :-)

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