gfni,avx512bw,avx512vland x86-64 only.
Performs a multiplication in GF(2^8) on the packed bytes. The field is in polynomial representation with the reduction polynomial x^8 + x^4 + x^3 + x + 1.
Uses the writemask in k - elements are zeroed in the result if the corresponding mask bit is not set. Otherwise the computation result is written into the result.