lib/raid6: Add AVX512 optimized gen_syndrome functions

Optimize RAID6 gen_syndrom functions to take advantage of
the 512-bit ZMM integer instructions introduced in AVX512.

AVX512 optimized gen_syndrom functions, which is simply based
on avx2.c written by Yuanhan Liu and sse2.c written by hpa.

The patch was tested and benchmarked before submission on
a hardware that has AVX512 flags to support such instructions

Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jim Kukunas <james.t.kukunas@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Megha Dey <megha.dey@linux.intel.com>
Signed-off-by: Gayatri Kammela <gayatri.kammela@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>