gccgo: accepts program with invalid identifier (2)

gccgo successfully compiles the following program:

``` go
package a
var _ *i
```

while gc and go/types reject it with:

```
go.go:2: undefined: i
go.go:2:7: undeclared name: i
```

gccgo must reject it too.
https://github.com/golang/go/issues/11522
