[x86] Add a feature bit: Fast_Unaligned_Copy

On AMD processors, memcpy optimized with unaligned SSE load is
slower than emcpy optimized with aligned SSSE3 while other string
functions are faster with unaligned SSE load.  A feature bit,
Fast_Unaligned_Copy, is added to select memcpy optimized with
unaligned SSE load.

	[BZ #19583]
	* sysdeps/x86/cpu-features.c (init_cpu_features): Set
	Fast_Unaligned_Copy with Fast_Unaligned_Load for Intel
	processors.  Set Fast_Copy_Backward for AMD Excavator
	processors.
	* sysdeps/x86/cpu-features.h (bit_arch_Fast_Unaligned_Copy):
	New.
	(index_arch_Fast_Unaligned_Copy): Likewise.
	* sysdeps/x86_64/multiarch/memcpy.S (__new_memcpy): Check
	Fast_Unaligned_Copy instead of Fast_Unaligned_Load.