Fortran: Allow IEEE_CLASS to identify signaling NaNs

Fortran: Allow IEEE_CLASS to identify signaling NaNs

We use the issignaling macro, present in some libc's (notably glibc),
when it is available. Compile all IEEE-related files in the library
(both C and Fortran sources) with -fsignaling-nans to ensure maximum
compatibility.

libgfortran/ChangeLog:

	PR fortran/82207
	* Makefile.am: Pass -fsignaling-nans for IEEE files.
	* Makefile.in: Regenerate.
	* ieee/ieee_helper.c: Use issignaling macro to recognized
	signaling NaNs.

gcc/testsuite/ChangeLog:

	PR fortran/82207
	* gfortran.dg/ieee/signaling_1.f90: New test.
	* gfortran.dg/ieee/signaling_1_c.c: New file.