gccgo: accepts program with invalid identifier

gccgo successfully compiles the following program:

```
package n
func _(t)
```

while gc and go/types say:

```
go.go:2: undefined: t
go.go:2:8: undeclared name: t
```

gccgo must reject this program.

gcc version 6.0.0 20150702 (experimental) (GCC)
