gccgo: can apply ^ to true

gccgo accepts the following program:

```
package a
var f = ^true
```

While gc and go/types say:

```
go.go:2:10: invalid operation: operator ^ not defined for true (untyped bool constant)
go.go:2: invalid operation: ^ untyped bool
```

gcc version 6.0.0 20150702 (experimental) (GCC)
