[patch, fortran] PR32704 - document -static-libgfortran

[FX Coudert]

> 2007-07-09  Daniel Franke  <franke.daniel@gmail.com>
>
> 	PR fortran/32704
> 	* invoke.texi: Document option -static-libgfortran.

OK. Sorry for not documenting it in the first place.

FX