emsdk

Support for tip-of-tree (tot) builds (#245)

emsdk install tot-upstream will install the very latest build from emscripten-releases. These builds are useful for emscripten github CI.

There isn't a latest or lkgr for emscripten-releases currently. What this does instead is get the git repo, and check if builds exist for the latest commits there, returning the latest of those.

This also makes us not update the emscripten-version.txt file if we are not an actual version. That is, for a tot build we leave that file unchanged in the emscripten checkout.

There is also a tot-fastcomp for fastcomp.