Fortran: Adjust handling of optional comma in FORMAT.

Fortran: Adjust handling of optional comma in FORMAT.

	This change adjusts the error messages for optional commas
	in format strings to give a warning at compile time unless
	-std=legacy is used. This is more consistant with the
	runtime library. A missing comma separator should not be
	encouraged as it is non-standard fortran.

	PR fortran/119586

gcc/fortran/ChangeLog:

	* io.cc: Set missing comma error checks to STD_STD_LEGACY.

gcc/testsuite/ChangeLog:

	* gfortran.dg/comma_format_extension_1.f: Update dg-options to
	"-std=legacy".
	* gfortran.dg/comma_format_extension_3.f: Likewise.
	* gfortran.dg/continuation_13.f90: Likewise.