[Emscripten] Use "-s MODULARIZE" to initialize module in an async style.

It seems like the latest Emscripten toolchain has a decision to remove the "--post-js" option, here we can use "-s MODULARIZE" to initialize module as a replacement. FYI:

modularize.cc
```C++
#include
#include

using namespace std;

int main (int argc, char **argv) {
cout << "Hello, Emscripten!" << endl;
return 0;
}

extern "C" int EMSCRIPTEN_KEEPALIVE add (int x, int y) {
return x + y;
}
```

modularize.html (generated by "emcc", need to revise)
```html
...

window.onload = function() {
MyModule().then(function(Module) {
console.log(Module['asm']['_add'](1, 2));
});
}

...
```

CMD:
```shell
sudo emcc modularize.cc -s WASM=1 -o modularize.html -O3 -s MODULARIZE=1 -s 'EXPORT_NAME="MyModule"'
```

*BTW, we can also use `--pre-js` param to append JavaScript code into the resulting script file.

Start by running the emcc command shown in the issue and inspect the generated modularize.html and module.js behavior. Compare the generated page with modularize.cc and verify that MODULARIZE initializes MyModule asynchronously and that calling _add(1, 2) produces 3. Done means the example works without relying on the removed --post-js option.

Written by the indexing model from the issue text.