tor-rtcompat: Dependency on x509-signature pulls in ring 0.16, breaks riscv64/loongarch64/...

Hello!

I have tor-rtcompat in my dependency tree and noticed I can't compile on some less-mainstream architectures anymore, because ring 0.16 fails to build (fwiw, ring 0.17 would work):

error: failed to run custom build command for `ring v0.16.20`

Caused by:
  process didn't exit successfully: `/home/kpcyrd/repos/apt-swarm/target/release/build/ring-b1e87b0ba2193236/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at /home/kpcyrd/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ring-0.16.20/build.rs:358:10:
  called `Option::unwrap()` on a `None` value
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...

This dependency is pulled in by x509-signature which has been archived, else I would've reported this over there.

The build issues could be fixed by either switching to a different crate, resuming development through either a fork or asking for crate ownership.