Last I used Go (granted, 2010), it refused to use the platform ABI, and shipped with kenc so that you could compile your C code for its ABI. So, it had C interop, but not binary interop with C. For the kind of interoperability you're talking about here, if each language uses its own ABI, we get O(N*N) complexity. Have things changed with the Go ABI, or at least its FFI?
I'm sure the Plan9 ABI variant used by Go makes more sense than the platform ABI, but supporting the platform ABI for FFI doesn't add much complexity at all to the compiler.
In that case, I stand corrected with Go (I incorrectly assumed that - being supposedly a systems language - it would support the platform ABI, but I guess Go is even worse than I thought ;) ).
I'm 91% sure my point still stands with Rust, though, at the very least if everything's compiled through LLVM (and uses the same calling conventions).
I'm sure the Plan9 ABI variant used by Go makes more sense than the platform ABI, but supporting the platform ABI for FFI doesn't add much complexity at all to the compiler.