This header doesn't build on g++ v6 as it's a C and not C++ header
(_Atomic is not a keyword in C++11). Since the C and C++ invocations
cannot be guaranteed to point to the same underlying atomic_int
implementation, remove support for them and use compiler intrinsics
instead.

BUG=webm:1461

Change-Id: Ie1cd6759c258042efc87f51f036b9aa53e4ea9d5