Update x86 fix-fp-int-compare-invalid.h for GCC 8.

Update x86 fix-fp-int-compare-invalid.h for GCC 8.

The glibc implementation of iseqsig relies on ordered comparison
operators raising the "invalid" exception for quiet NaN operands, with
a workaround on platforms where a GCC bug means that exception is not
raised.  For x86, that bug has now been fixed for GCC 8, so this patch
disables the workaround in that case.  If and when the corresponding
bugs for powerpc and s390 are fixed, the headers for those platforms
should of course be updated similarly.

Tested for x86_64 and x86, including with GCC mainline.  Note that
other failures appear with GCC mainline because of spurious use of
ordered comparison instructions for unordered operations
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82692>.

	* sysdeps/x86/fpu/fix-fp-int-compare-invalid.h
	(FIX_COMPARE_INVALID): Define to 0 if [__GNUC_PREREQ (8, 0)].