Google supposedly has implemented something similar called user-level threads, which they talked about at the 2013 Linux Plumbers Conference. If I recall the video correctly, they talk about plans to open source it, but as far as I know there hasn't been much movement in that space yet:

http://www.linuxplumbersconf.org/2013/ocw/proposals/1653 https://www.youtube.com/watch?v=KXuZi9aeGTw