Make emscripten_get_now() work when running in SpiderMonkey shell. 

See https://developer.mozilla.org/en-US/docs/SpiderMonkey/Hacking_Tips

I want to run MathGeoLib benchmarks through IonMonkey-enabled SpiderMonkey, and emscripten_get_now() fails there (no object window). Calling Date.now() gives only millisecond-precision, dateNow() returns sub-millisecond precision.

This pull request hasn't been tested at all in other environments. Is ENVIRONMENT_IS_SHELL the appropriate mechanism to detect SpiderMonkey?