WIP: Add Emscripten Toolchain

[chkuendig]

This is based on the discussion on discord and scummvm/scummvm#6827.
I tried to continue this based on @ccawley2011's old branch.

For me I managed to properly build scummvm with the following setup:

create docker container:

• edit Makefile.user and set both TOOLCHAINS_ENABLED := common emscripten and TOOLCHAINS_BUILT := common emscripten
• run make toolchains
• run docker run -v ../scummvm:/source --name toolchain_emscripten -i -t toolchains/emscripten /bin/bash

build scummvm (from within the container):

cd /source && \
emconfigure ./configure  --host=wasm32-unknown-emscripten --build=wasm32-unknown-emscripten \
  --enable-verbose-build --disable-all-engines --enable-engine=testbed,grim \
  --enable-zlib --enable-png --enable-jpeg --enable-vorbis --enable-gif --enable-freetype2 \
  --enable-mpeg2 --with-mpeg2-prefix=/usr/local/emscripten/scummvm-libs/lib/ \
  --enable-a52 --with-a52-prefix=/usr/local/emscripten/scummvm-libs/lib/ \
  --enable-faad --with-faad-prefix=/usr/local/emscripten/scummvm-libs/lib/ \
  --enable-theoradec --with-theoradec-prefix=/usr/local/emscripten/scummvm-libs/lib/  && \
  emmake make dist-emscripten

there's a few things left todo to get all libs tested and working, but the primary issue I'm facing is that I'm not 100% how to now make that work in buildbot to trigger builds.