More accurate explanation of wasm32 state in readme (#776) There are three main wasm32 target triples: * wasm32-unknown-unknown * wasm32-unknown-emscripten * wasm32-unknown-wasi The wasm32 port is only for the wasm32-unknown-emscripten target triple. (The other triples have no dynamic linking support so libffi would be both hard to port and of limited utility.)