gccgo: does not recognize unused import

gccgo successfully compiles the following program:

``` go
package a
import  _ "fmt"
import "fmt"
```

Both gc and gotype say:

```
"fmt" imported but not used
```

They seem to be right.
gofrontend is on commit 81810917af7ba19e1f9f8efc8b1989f7d6419d30
