contrib: ffmpeg: switch to a more recent x86_64 CPU architecture

In VLC 4 we use __builtin_clz() since a1245add.

But it's turned off for some old CPU architectures in FFmpeg ¹.

core2 has support for builtin clz used by VLC. It should also be the minimum architecture for Intel mac. It also has sse3, unlike athlon64.

1. https://code.ffmpeg.org/FFmpeg/FFmpeg/src/branch/release/7.1/configure#L6485 ↩