gccgo: does not understand greek capiltal letter yot

gccgo fails to compile the following program:

```
package p
const Ϳ = 0
```

```
go.go:2:7: error: invalid character 0x37f in input file
```

That's greek capiltal letter yot:
http://www.fileformat.info/info/unicode/char/37f/index.htm

gcc version 6.0.0 20150702 (experimental) (GCC)
