avutil: vector and serialize CPU flags initialization

avutil: vector and serialize CPU flags initialization

av_set_cpu_flags_mask() is not thread-safe. So are the first call to
av_get_cpu_flags() after loading libavutil, and the first call to
av_get_cpu_flags() after a call to av_set_cpu_flags_mask(). So call
those under the avcodec lock during initialization.