(libretro) Use new logging interface, Emscripten port

This is a logging interface added to the libretro API. It should allow for crossplatform logging.

Also in this pull request is Emscripten port support. We have a Javascript version of Genesis Plus GX running through RetroArch running right now using this LLVM-to-JavaScript compiler. You can run it in either Chrome or Firefox.

http://toadking.com/retroarch/genesis_plus_gx.html