This changes the emscripten package so that it specifies the rev from
the binaryen repo to use, and sets it to always use the version that has
been tagged for use with that version of emscripten. This should force
future updates of emscripten to also update binaryen.
Binaryen can also be installed as a stand-alone package, so there's some
logic added to the binaryen package to allow building in both ways, and
distinguishing between them.
497e50cf16