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

[Sam Elliott via llvm-commits]

lenary wrote:

How do the other backend tags work? Do they just look for the backend name in the changed paths, or are they more picky?



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