lib/raid6/test/Makefile: Add avx512 gen_syndrome and recovery functions

Adding avx512 gen_syndrome and recovery functions so as to allow code to
be compiled and tested successfully in userspace.

This patch is tested in userspace and improvement in performace is
observed.

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>