Latest emscripten support, Canon Cat system

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.

Parameters were tested only with Coleco (a single game) and Canon Cat.  Additional system and game testing should be done before accepting this pull request.

Also, someone still has to change the emscripten submodule to point to the latest version if this request is to be accepted.