Enable source maps when targeting asmjs in debug mode

[Thinkofname]

This is emcc's -g4 flag. https://kripken.github.io/emscripten-site/docs/tools_reference/emcc.html#emcc-g

Source maps make debugging asmjs code much easier with file name and line number information. I have no idea if this option works for wasm as well.

[brson]

Thanks for the suggestion! Seems worth a shot.