Support AVX512F about raxml-ng HOT 6 OPEN

This certainly made sense when this testing likely happened. There have been a few things that have changed recently, including much more RAM bandwidth (in cases where that was bottlenecking the system), and much more efficient AVX512 hardware (including more on-die resources and optimizations) which might warrant re-running some of them. I guess it's a next-next generation RAxML idea, perhaps? I know ExaML was at least 3 times faster on KNL back in the day, but since then no system has treated double-precision AVX512 as a first class citizen again until Zen 4 and IceLake/Sapphire Rapids.

https://www.phoronix.com/review/intel-sapphirerapids-avx512

The above link shows over time the profound differences (including reduced penalties and proportionately greater performance) over time for essentially the same AVX512 code. A modern 12-channel 4800MHz DDR5 Genoa system or a new 60-core Sapphire Rapids system seems to net proportionately more performance from AVX512 in double-precision floating-point math than the sad old Skylake/Cascade Lake Xeons (which were indeed bottlenecked in many ways).

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Despite extensive experiments, we never managed to get reasonable speedups with AVX512 compared to AVX2. Hence, AVX512 support was never integrated to RAxML-NG.

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@GabeAl: you're right, it mostly concerns libpll, and we do have it on our radar anyway. Still, please feel free to open a corresponding issue in the (new) libpll repository:

https://github.com/xflouris/libpll

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May I know how exactly to compile with AVX512? eg: cmake -DENABLE_AVX512=TRUE ?
Thanks.

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@GabeAl thanks for the heads up!

Good to know that AMD added AVX512 support as well, and Genoa benchmark results look really impressive!

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@amkozlov @GabeAl thanks a lot, both of you. I see. I will consider AMD's CPU next time if my budget can cover them. :)

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