webassembly: runtime not being exited

emscripten recently changed NO_EXIT_RUNTIME default to 1, meaning the runtime is not exited.

https://kripken.github.io/emscripten-site/docs/getting_started/FAQ.html#what-does-exiting-the-runtime-mean-why-don-t-atexit-s-run

"Which means that we don't include code to shut down the runtime. That means that when main() exits, we don't flush the stdio streams, or call the destructors of global C++ objects, or call atexit callbacks."

If you are running several Qt Webassembly runetimes (such as with testing), the browser will eventually run out of memory.

We need to add -s NO_EXIT_RUNTIME=0 to link line.