x86-64: Use ZMM16-ZMM31 in AVX512 memset family functions

Update ifunc-memset.h/ifunc-wmemset.h to select the function optimized
with AVX512 instructions using ZMM16-ZMM31 registers to avoid RTM abort
with usable AVX512VL and AVX512BW since VZEROUPPER isn't needed at
function exit.

(cherry picked from commit 4e2d8f35)