fortran [PR122957] DTIO incompatibility with -fdefault-interger-8

fortran [PR122957] DTIO incompatibility with -fdefault-interger-8

The -fdefault-integer-8 option is optional to assist with legacy
fortran codes. It is not a Standard requirement and is not
compatible with the newer user defined derived type I/O.

	PR fortran/122957

gcc/fortran/ChangeLog:

	* interface.cc (gfc_match_generic_spec): Issue an error
	so that users do not use -fdefault-integer-8 with DTIO.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr122957.f90: New test.