[Patch,fortran] PR 27740 Symbol versioning for libgfortran

[FX Coudert]

> I'm not a big fan of undocumented options.  People have a way of
> finding and using these options.  I'd rather document the option
> with something like
>
> -fallow-leading-underscore -- This option is intended for internal
>     use by the gfortran developers.  IT IS SUBJECT TO GO AWAY WITHOUT
>     WARNING.

After thinking some more, I do agree with Steve. The options should  
not be output by gfortran -v (hence, they should be marked as  
Undocumented in the lang.opt file), but the invoke.texi should  
mention that they're for internal use only.

I'll write it before we do the commit.

FX