postRun hooks not run when exit runtime is enabled

This issue can be reproduced with the following files.

**pre-js.js**
```js
Module = {};
Module['postRun'] = function() {
console.log('Hello, this is postRun.');
}
```

**main.c**
```C
main() {}
```

And the commands:

1. `emcc main.c --pre-js pre-js.js -o no-exit.js`

2. `emcc main.c --pre-js pre-js.js -o exit.js -s EXIT_RUNTIME=1`

`no-exit.js` runs the hook, but `exit.js` does not, which is extremely counterintuitive.

The reason is that postRun hooks are run after `callMain` has returned and `callMain` calls `exit`, which returns if the exit runtime is disabled but kills the interpreter otherwise. One potential solution is to move the postRun hook execution into `exit`, but I don't know if that would break anything. A better solution would be to remove the postRun hooks entirely, but idk if anyone depends on them.