Add a compat stdatomic.h implementation based on pthreads

Adapted from the code by Rémi Denis-Courmont from VLC

This merges libav commit f9a6a80e.

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>