Avoid C++ tests when the C++ cannot be linked

	* elf/Makefile [ifeq (yes,$(build-shared)) (tests)] (tst-nodelete):
	Add iff $CXX is set.
	[ifeq (yes,$(build-shared)) (modules-names)] (tst-nodelete-rtldmod):
	Likewise.
	(tst-nodelete-zmodiff): Likewise.
	* nptl/Makefile [[ifeq (,$(CXX))] (tests-unsupported)]: Add tst-once5.