s390: arch15: Prepare for future builtins

gcc/ChangeLog:

	* config/s390/s390-builtins.def (B_VXE3): Define.
	(B_ARCH15): Define.
	* config/s390/s390-c.cc (s390_resolve_overloaded_builtin):
	Consistency checks for VXE3.
	* config/s390/s390.cc (s390_expand_builtin): Consistency checks
	for VXE3.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: VXE3 effective target check.