[PATCH, fortran] Add gfc_define_builtin_with_spec

[FX]

>> If unused on trunk, why would we commit it there?
>> When your branch is merged, you'll merge it along. Otherwise that defeats the purpose of working on a branch, unless I misunderstand something...
> 
> This patch is not branch-specific.

As far as I understand, we don’t add unused code to the codebase. A quick grep through the current front-end and middle-end confirms that.
Because it’s not used and not tested, it could bitrot and adds to the maintainance cost.

FX