[libc++] decoupling Freestanding atomic<T> from libatomic.a

This patch introduces non-lockfree atomics that do not require using
an external libatomic. This work is done with the long-term goal of
allowing the use of <atomic> in freestanding environments.

Thanks to Olivier Giroux for the patch.
Differential Revision: https://reviews.llvm.org/D56913

llvm-svn: 355318