Disable auto vectorization of xxhash64, when AVX512 is present. about zstd

[TocarIP]

Someone asked me about AMD offline so might as well post here. On genoa (Zen4) AVX512 is actually ~30% faster ( 1135871 ns/op vs 885935 ns/op).

[TocarIP]

Regression on Intel is bigger than gain on AMD, so disabled by default makes sense. Using the same XXH_ENABLE_AUTOVECTORIZE to allow opt-in is probably the best.

[Cyan4973]

We already disable vectorization for xxhash32 if sse4 is present, so we should probably do the same for xxhash64 and avx512.

Indeed,
this seems like the same issue,
and we should probably use a similar solution.

[Cyan4973]

OK, so it makes the situation a bit less clear,
since avx512 is now sometimes beneficial, sometimes detrimental.
It probably opens the door towards offering a user choice on this topic.

However, it doesn't remove the question of "what's a good default",
and if I read the situation correctly, it seems that disabling avx512 for xxh64 remains a reasonable default at this point in time.

[Cyan4973]

Cyan4973/xxHash#924