[Patch, Fortran] PR54339 - Update gfortran.texi's standard refs, status and TS29113 sections

[Dominique Dhumieres]

Tobias,

AFAICT, the links in

 Additionally, the GNU Fortran compilers supports the OpenMP specification
 (version 3.1, @url{http://openmp.org/@/wp/@/openmp-specifications/}).
@@ -545,6 +546,10 @@ for them, which work with GNU Fortran.  They can be found at
 @uref{http://www.fortran.com/@/iso_varying_string.f95} and at
 @uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/ISO_VARYING_STRING/}.

are all broken.

TIA

Dominique