Signature of method taking generic array differs from Scala 2

## minimized code

```Scala
class C {
  def f[A](as: Array[A]) = 42
}
```

## Compilation output

```scala
$ ~/bin/dotc vargen.scala && javap C
Compiled from "vargen.scala"
public class C {
  public C();
  public <A> int f(A[]);
}

```

## expectation
```scala
Compiled from "vargen.scala"
public class C {
  public <A> int f(java.lang.Object);
  public C();
}
```
