Add codegen-{cranelift,gcc} marker teams for custom codegen backend ping groups

[jieyouxu]

Part of rust-lang/rust#138116.
Discussion: #t-infra > Triagebot ping groups for GCC and cranelift backends.
rust-lang/rust side triagebot configuration + docs: rust-lang/rust#138120

cc @GuillaumeGomez @bjorn3 @antoyo

[Urgau]

Why are we creating marker teams? We already have a wg-gcc-backend working group.

I don't see one for cranelift, but we should do the same and create one instead of creating marker teams.

[jieyouxu]

Why are we creating marker teams? We already have a wg-gcc-backend working group.

For ping groups. You can't ping rust-lang/wg-gcc-backend if u're not an org member.

[Urgau]

Oh, the llvm one is actually for icebreakers-llvm and not wg-llvm. Fair enough.

I still wish we didn't have to create marker teams.

[jieyouxu]

Oh I raced with ur comment here over at the r-l/r side PR lol.

Yeah, I don't think you can use a real wg team as ping group.