gccgo: knowns unknown escape sequence

gccgo successfully compiles the following program:

``` go
package a
var e = "\X00"
```

While gc and go/types say:

```
go.go:2: unknown escape sequence: X
go.go:2:11: unknown escape sequence
```

The spec allows only lower-case x for hex-encoded chars.

gcc version 6.0.0 20150702 (experimental) (GCC)
