x86: Optimize less_vec evex and avx512 memset-vec-unaligned-erms.S

No bug. This commit adds optimized cased for less_vec memset case that
uses the avx512vl/avx512bw mask store avoiding the excessive
branches. test-memset and test-wmemset are passing.

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>