Support --enable-hardcoded-path-in-tests in benchtests

benchtests should use $(test-via-rtld-prefix) and $(+link-tests) like
other glibc tests.

	[BZ #19783]
	* benchtests/Makefile (run-bench): Replace $(rtld-prefix) with
	$(test-via-rtld-prefix).
	($(binaries-bench)): Replace $(+link) with $(+link-tests).