Fortran: fix testcase compiler flags

-fsignaling-nans is already passed by ieee.exp, so it's not needed.
We must use dg-additional-options instead of dg-options, otherwise we
override flags passed from ieee.exp. And we need to use -w because
some options only make sense for the Fortran source.

gcc/testsuite/ChangeLog:

	* gfortran.dg/ieee/signaling_1.f90: Adjust flags.

gcc/testsuite/gfortran.dg/ieee/signaling_1.f90

@@ -1,7 +1,9 @@
 ! { dg-do run }
 ! { dg-require-effective-target issignaling } */
 ! { dg-additional-sources signaling_1_c.c }
-! { dg-options "-fsignaling-nans" }
+! { dg-additional-options "-w" }
+! the -w option is needed to make f951 not report a warning for 
+! the -fintrinsic-modules-path option passed by ieee.exp
 !
 program test
   use, intrinsic :: iso_c_binding