This updates JSMESS with tested and working EMCC parameters
for the latest version of emscripten, and adds a tested and
working system makefile for the Canon Cat (although the
screen size seems to be incorrect, and I don't know what
the -nokeepaspect parameter affects).

It also removes the forcible abort() and lets emscripten
launch the MESS main loop on its own.

Note that this commit IS NOT ENOUGH. You still have to
change the emscripten submodule to point to the latest
version, or fetch it manually.