Fix #3668

Fix #3668

Following the implementation in Google's cpu_features tool made by an
Intel engineer used for original implementation, we only check for the
presence of the second AVX-512 FMA unit if the CPU supports AVX-512F
(foundation), which should be present for all AVX-512 implementations.

That's distinct from the way cpu_features tool does AVX detection, but
should be no less reliable.