Fix AVX512BW Fletcher code on AVX512-but-not-BW machines

Introduce a specific valid function for avx512f+avx512bw (instead
of checking only for avx512f).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam Moss <c@yotes.com>
Signed-off-by: Romain Dolbeau <romain@dolbeau.org>
Closes #11937
Closes #11938