@:nativeGen interface use leads to crash

When investigating https://github.com/HaxeFoundation/haxe/issues/10427, I discovered that this program using native interfaces crashes:

```haxe
@:nativeGen
interface IFirst {
public function doFirst():Void;
}

@:nativeGen
interface ISecond {
public function doSecond():Void;
}

class Main implements IFirst implements ISecond {
public var data:Array = [1, 2, 3];

public function new() {}

public function doFirst() {
trace(data);
}

public function doSecond() {
trace(data);
}

static function main() {
final obj = new Main();

final interfaceRef:ISecond = obj;

cpp.vm.Gc.run(true);

interfaceRef.doSecond();
}
}
```

Compiled on linux with:
```
haxe -m Main --cpp bin
```

This crashes while trying to join the Array because the `->length` field has become garbage.

Start by compiling the supplied Main program on Linux with `haxe -m Main --cpp bin` and reproduce the crash after `cpp.vm.Gc.run(true)`. Then inspect the generated C++ around the ISecond dispatch and Array access; done means the program completes without the Array length becoming corrupted.

Written by the indexing model from the issue text.