gccgo: can do bitwise or on fp constants

gccgo successfully compiles the following program:

``` go
package a
const d rune = 1e2|2
```

gc and go/types reject this program saying:

```
illegal constant expression: untyped number | untyped number
invalid operation: operator | not defined for 1e2 (untyped float constant 100)
```

gcc version 6.0.0 20150702 (experimental) (GCC)
