Distribute GCC backend about rustup

[rami3l]

@GuillaumeGomez Hi, and thanks a lot for all the work!

I'm very excited to see yet another codegen backend becoming available to end users!

Looking at how rustc-codegen-cranelift got shipped in rust-lang/rust#81746, it looks like distributing a new component with Rustup is more about ensuring that the right manifests and artifacts are on the release server, and probably has little to do with the Rustup codebase (except for a few things like the rustup doc command where Rustup itself is responsible for consuming the component).

I think you can start the experiment as long as you're sure that your component has a -preview suffix and is exclusively available on nightly, which is enough to show that it's still experimental. Again, just follow rust-lang/rust#81746.

Currently I'm not aware of any special handling regarding codegen backends on Rustup's side, and given that this time some other party like Cargo is responsible for consuming this new component, I guess there's nothing to be changed from Rustup, but we'll have to wait and see. I'll keep this issue open just in case.