It does need a wasm platform target. I'm not sure the triple but will know (I'd expect wasm-unknown-emscripten for wasm + the emscripten runtime).
The compiler already has a target for asm.js, asmjs-unknown-emscripten. This target sets
target_pointer_width = "32",
target_os = "emscripten",
target_arch = "asmjs". For wasm the target spec can probably be exactly the same except that
target_arch = "wasm".
The best way for mir2wasm to proceed for now is probably to create its own target spec and pass that to the session config (or wherever it goes).