libgfortran: Use __builtin_issignaling in libgfortran [PR105105]

libgfortran: Use __builtin_issignaling in libgfortran [PR105105]

The following patch makes use of the new __builtin_issignaling,
so it no longer needs the fallback implementation and can use
the builtin even where glibc provides the macro.

2022-08-26  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/105105
	* ieee/ieee_helper.c: Don't include issignaling_fallback.h.
	(CLASSMACRO): Use __builtin_issignaling instead of issignaling.
	* ieee/issignaling_fallback.h: Removed.