Detect the compiler's support of C11 atomics, and use C11 atomics when
available.

Note that libyuv::MaskCpuFlags() is still not thread-safe.

BUG=libyuv:641
TEST= cpu_thread_test.cc adds a pthread based test
R=wangcheng@google.com

Change-Id: If05b1e16da833105a0159ed67ef20f4e61bc7abd
Reviewed-on: https://chromium-review.googlesource.com/510079
Commit-Queue: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>