apply delayed side effects before using camlp4

Camlp4 built-in extensions and some others are using Camlp4.Register instead of performing side-effects at toplevel.  The default camlp4 binary calls this code after loading each syntax extension:

```ocaml
  Camlp4.Register.iter_and_take_callbacks 
    (fun (name, callback)-> callback ())
```

It is also called by `Camlp4Top` which is linked in `camlp4o.cma`. To be safe it should be inserted after syntax extension. With the following simple patch extensions using `Camlp4.Register` will be applied after the others.