emsdk

Add emscripten-releases support for release tags (#243)

* Structures emscripten-releases-tags.txt to mention the latest release, and then a list of all previous releases.
* Adds a new release for 1.38.33.
* If the manifest has emscripten_releases_hash, then we write out emscripten-version.txt with the proper version. We start with the hash, and check if there is a released version for it, if so we emit that, otherwise we emit the hash.
* Windows zipfile fix.