Added memset optimized with AVX512 for KNL hardware.

It shows improvement up to 28% over AVX2 memset (performance results
attached at <https://sourceware.org/ml/libc-alpha/2015-12/msg00052.html>).

    * sysdeps/x86_64/multiarch/memset-avx512-no-vzeroupper.S: New file.
    * sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Added new file.
    * sysdeps/x86_64/multiarch/ifunc-impl-list.c: Added new tests.
    * sysdeps/x86_64/multiarch/memset.S: Added new IFUNC branch.
    * sysdeps/x86_64/multiarch/memset_chk.S: Likewise.
    * sysdeps/x86/cpu-features.h (bit_Prefer_No_VZEROUPPER,
    index_Prefer_No_VZEROUPPER): New.
    * sysdeps/x86/cpu-features.c (init_cpu_features): Set the
    Prefer_No_VZEROUPPER for Knights Landing.