We only use `_LIBCPP_DISABLE_EXTENSION_WARNINGS` in a single place while
we use extensions all over the place. The warnings are already disabled,
since libc++'s headers are system headers, so this shouldn't be in any
way observable by users.

This patch documents the underlying API for implementing atomics on a
platform.

This doesn't change the operations that std::atomic is based on, but it
reorganizes the C11 / GCC implementation split to make it clearer what's
the base support layer and what's not.