This makes it possible to tell the emsdk to get "latest-upstream", which fetches the latest lkgr from there. This will probably be a common use pattern, I expect we may want to recommend users start trying out the wasm backend that way soon. This will also let us simplify this code: https://github.com/kripken/emscripten/blob/incoming/.circleci/config.yml#L334
Aside from adding the "latest-upstream" alias, this PR has
* A few minor cleanups in the emsdk code.
( Minor restructuring of how we define the upstream stuff in the manifest: It seemed odd to have 3 things (clang, emscripten, binaryen) that are all coming from a single archive from the waterfall. Simpler to have just one - the archive is one big lump, there's no way to download just part of it.
* Also add node 8.9.1 as a dependency of the upstream sdk, which makes things usable out of the box (node.js is the one thing not provided by the waterfall archive).
