[wasm-gc] physical_equal() should not be allowed with extern types with wasm-gc backend

[jayphelps]

At least on the wasm-gc backend, moonbit allows it, but you get a runtime validation error (that most non-compiler folks won't understand). It feels like ideally moonbit would detect this and emit an error. I stubbled upon this by mistake as I forgot externref's cannot be ref.eq checked.

In the grand scheme of things this seems extremely low priority right now (e.g. backlog it) but sharing anyway for tracking purposes.