Add gcc compiler links with universal, un-versioned names

Of course it all remains in the cellar (gcc links itself to `opt`), so there should be no conflicts with alternative versions. This will simplify the use of gcc compilers or linkers without relying on a specific version number. If a formula needs for some reason to depend on the latest gcc, it will now be simpler:

```
depends_on 'homebrew/dupes/gcc' if build.include? "some-option"
...
ENV["CXX"] = Formula.factory('homebrew/dupes/gcc').opt_prefix/"bin/g++"
```

Previously, this variable would be version dependent because of the compiler suffix. From what I can tell, the version suffix is not technically necessary anymore now that gcc is installed in the opt directory, but perhaps I misunderstand the purpose of this.