> With GCC adding Rust support there is no longer one "official" compiler.

No, there's will still only be one official compiler – the one written and provided by The Rust Project.

> If Rust wants to supplant C in embedded, a specification is required.

You're getting your history backwards.

C did not become successful because of its specification, it got a specification because it was successful.

Similarly, if Rust becomes successful enough, it will probably get a specification. But the popularity will drive the work on a specification, not the other way around.