Any pre-C++11 clients of stdatomic.h that use libc++ are being forced
over to <atomic>, which they don't have the language support to use.
Bug:17736764
Change-Id: I62445c1f2541410a1569498c09433c7196635537
(cherry picked from commit 3ce0769aa5f9a991af1d167f730d987dd002253c)