cmd/compile: accepts invalid function type

gc successfully compiles the following code:

``` go
package a
var f func((i), _ int)
```

both gccgo and gotype reject it saying:

```
error: invalid named/anonymous mix
expected identifier
```

Spec does not allow parenthesis there.
go version devel +be2a3e2 Thu Jul 2 08:29:33 2015 +0000 linux/amd64
