How about stabilizing llvm_asm!("<arch>", <current_syntax>) as a function that returns true if the arch is correct and the backend supports this particular syntax (as specified by LLVM) and false otherwise? (as well as check_llvm_asm! that only checks but does nothing)
This requires almost no effort to implement in rustc, and allows Cranelift to implement any subset of the LLVM assembly syntax, returning false for anything else and letting the generic fallback code (that is already needed to support unknown architectures) handle it.