Change our method of calling main() in emccpre.js.

Change our method of calling main() in emccpre.js.

I've just upgraded to emcc 1.16.0, in which something fiddly has
happened to the semantics of Module.run() vs noInitialRun - now
setting the latter seems to cause the former to do everything except
calling main(), and then refuse to ever do anything again. So now I
have to use Module.callMain() in place of Module.run() when I finally
do get round to wanting to call main().

[originally from svn r10180]