Add ifunc tests for x86_64 memset_chk and memset

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.