[php, python] big int issue

the following test in Test.hx failed when targeting php and python:

``` haxe
assertScript("0xBFFFFFFF", 0xBFFFFFFF);
```

The same error is returned by php and python:

```
ERR: Test.hx:16(Test.assertScript) - expected '-1073741825' but was '3221225471'
```
