Support AVX512 for likelihood function

[GabeAl]

Similar to enhancement request on the old tracker from 2013: https://app.assembla.com/spaces/phylogenetic-likelihood-library/tickets/72-port-likelihood-function-xeon-phi/details#

Cross-posted from: https://github.com/amkozlov/raxml-ng/issues/16

Cheers! Gabe