Have stdatomic.h punt to C++ atomic when possible

Have stdatomic.h punt to C++ atomic when possible

This is an alternate, somewhat simpler, fix that makes it safe to
include both <atomic> and <stdatomic.h> from C++ code in either order.
It means that C code consistently uses one implementation of atomics
and C++ another.  We still have to make sure that those two
implementations interoperate correctly at runtime; in particular,
any flavor of atomic object needs to be represented exactly like the
underlying type, with the proper alignment constraint.

Bug:17007799
Change-Id: Iffcfc5220d8fa150f89dd083a121b24d23f268fc
(cherry picked from commit 019d3958118b7dc3ec8444ad2accca50c268b737)