Fortran: downgrade use associated namelist group name to legacy extension

Fortran: downgrade use associated namelist group name to legacy extension

The Fortran standard disallows use associated names as namelist group name
(e.g. F2003:C581, but also later standards).  This feature is a gfortran
legacy extension, and we should give a warning even for -std=gnu.

gcc/fortran/ChangeLog:

	* match.cc (gfc_match_namelist): Downgrade feature from GNU to
	legacy extension.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr88169_3.f90: Adjust pattern.