[Build Moot] Migrate emscripten/emsdk out of buildroot into the flutter engine repo

As discussed today in Flutter Build Moot (https://github.com/flutter/flutter/issues/67373).

Emscripten is currently stored in the buildroot under `//buildtools`. This needs to be moved to the flutter engine repo.