[cpp] @:native on static fields unusable for output

Unlike all other targets cpp treats `@:native` meta on static fields as global path. And this blocks ability to rename output names for class static fields. Example:

```haxe
class Foo {
@:native('Baz')
public static function bar(): Void {}
}

class Main {
public static function main() {
Foo.bar();
}
}
```

In all other targets this works, but in cpp it produces problematic code. In `Foo.h` you will find `static void _hx_Baz();`, in `Foo.cpp` - strange `void Foo::_hx_Baz() { (void)::Baz(); }` and in `Main.cpp` - simply `Baz();`.

In my opinion in ideal world, cpp should have been consistent with others and should have treated a path as global only if it started with `::` or empty space.

In real world to fix this and not to introduce breaking change, cpp needs to treat `@:native` on extern classes and outputted classes differently (global for former and relative for latter).