This patch adds ifunc tests for x86_64 memset_chk and memset. It also
defines HAS_AVX2 with AVX2_Usable since AVX2 may not be usable even if
processor has AVX2.
* sysdeps/x86_64/multiarch/ifunc-impl-list.c (__libc_ifunc_impl_list):
Add tests for memset_chk and memset.
* sysdeps/x86_64/multiarch/init-arch.h (HAS_AVX2): Defined
with AVX2_Usable.