Detect if AVX2 is usable

This patch checks and sets bit_AVX2_Usable in __cpu_features.feature.

	* sysdeps/x86_64/multiarch/ifunc-defines.sym (COMMON_CPUID_INDEX_7):
	New.
	* sysdeps/x86_64/multiarch/init-arch.c (__init_cpu_features):
	Check and set bit_AVX2_Usable.
	* sysdeps/x86_64/multiarch/init-arch.h (bit_AVX2_Usable): New
	macro.
	(bit_AVX2): Likewise.
	(index_AVX2_Usable): Likewise.
	(CPUID_AVX2): Likewise.
	(HAS_AVX2): Likewise.