Fortran: Fix test on targets without REAL128

REAL128 is a named constant, so we cannot simply use
(REAL128 > 0) to conditionally compile for targets with
REAL128.

gcc/testsuite/ChangeLog:

	PR fortran/89639
	* gfortran.dg/ieee/ieee_9.f90: Adjust test for targets without
	REAL128.