avutil/cpu: Fix race condition in av_cpu_count()

avutil/cpu: Fix race condition in av_cpu_count()

av_cpu_count() intends to emit a debug message containing the number of
logical cores when called the first time. The check currently works with
a static volatile int; yet this does not help at all in case of
concurrent accesses by multiple threads. So replace this with an
atomic_int.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>