gccgo: accepts invalid UTF-8

gcc compiles the following program, while gc and go/types reject it:

``` go
package G
//line \FF:8
```

(that \FF needs to be converted to actual FF byte)
gc and go/types say:

```
go.go:2:8: illegal UTF-8 encoding
go.go:2: illegal UTF-8 sequence ff 3a
```

Compilers should agree on validity of this program.

gcc version 6.0.0 20150702 (experimental) (GCC)
