[eval] debugger doesn't use toString() representation for Int64

The example from #9036 now works, but the following is still broken:

```haxe
import haxe.Int64;

class Main {
static function main() {
var i:Int64 = 0;
trace(i); // 0
}
}
```

You just get a very ugly-looking `haxe._int64.___Int64` in the debugger:


Not sure if it's a general issue for abstracts or just for `Int64`.