testsuite: Fix gfortran.dg/ieee/comparisons_3.F90 testsuite failures

The testcase should use dg-additional-options istead of dg-options to
not overwrite default compile flags that include path for finding
the IEEE modules.

gcc/testsuite/ChangeLog:

	* gfortran.dg/ieee/comparisons_3.F90: Use dg-additional-options
	instead of dg-options.