Don't use an STL for libm.

The default is none right now, but this will be needed when the
default becomes libc++.

Change-Id: Ib022adac2b9dc08fbfae830908128043cfffb91e
Only use <atomic> for C++11 and newer.

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.

Change-Id: I62445c1f2541410a1569498c09433c7196635537
Merge "Don't use an STL for libm."
Merge "Only use <atomic> for C++11 and newer."