IBM Z: Fix PR96456

The testcase failed because our backend refuses to generate vector
compare instructions for signaling operators with -fno-trapping-math
-fno-finite-math-only.

gcc/ChangeLog:

	PR target/96456
	* config/s390/s390.h (TARGET_NONSIGNALING_VECTOR_COMPARE_OK): New
	macro.
	* config/s390/vector.md (vcond_comparison_operator): Use new macro
	for the check.

gcc/testsuite/ChangeLog:

	PR target/96456
	* gcc.target/s390/pr96456.c: New test.