Switch benchmarks and tests to libc++.

Change-Id: I3b9c8f8c14a1e227e67ae1977cc29b0e8fccef5d
diff --git a/tests/stdatomic_test.cpp b/tests/stdatomic_test.cpp
index b7fb19b..eb030bf 100644
--- a/tests/stdatomic_test.cpp
+++ b/tests/stdatomic_test.cpp
@@ -14,8 +14,10 @@
  * limitations under the License.
  */
 
-#include <stdatomic.h>
 #include <gtest/gtest.h>
+// Fool stdatomic.h into not using <atomic>.
+#undef _USING_LIBCXX
+#include <stdatomic.h>
 #include <pthread.h>
 #include <stdint.h>