Add Emscripten to supported platforms

[UebelAndre]

Creating this issues to spread awareness. Seems like there's a toolchain for it now? https://github.com/emscripten-core/emsdk/tree/master/bazel

I don't know how much work it is but seems like Emscripten could be added to the list of support platforms.

[UebelAndre]

References to emscripten were added in #3507 and at a glance seems like it all would work. Closing this out for now.