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]