[infra] Exclude libc config files from RISC-V label (#205407)
Currently, almost every single new function in libc is tagged with
`backend:RISC-V` because the configurations in `libc/config/**/riscv`
are changed when adding a function.
The intention of this PR is to exclude only changes to `libc/config`
which touch `riscv` paths from being tagged. There are other
files/directories in libc which contain risc-v specific code, which
should continue to be tagged with `backend:RISC-V`.
I'm not 100% sure of the syntax here, but I based this PR on this
example in the labeller readme:
https://github.com/actions/labeler/blob/f612d9ad188e81643862c2de70f57fbb1d17abd1/README.md?plain=1#L141-L145