x86-64: Fix memcpy IFUNC selection

Chek Fast_Unaligned_Load, instead of Slow_BSF, and also check for
Fast_Copy_Backward to enable __memcpy_ssse3_back.  Existing selection
order is updated with following selection order:

1. __memcpy_avx_unaligned if AVX_Fast_Unaligned_Load bit is set.
2. __memcpy_sse2_unaligned if Fast_Unaligned_Load bit is set.
3. __memcpy_sse2 if SSSE3 isn't available.
4. __memcpy_ssse3_back if Fast_Copy_Backward bit it set.
5. __memcpy_ssse3

	[BZ #18880]
	* sysdeps/x86_64/multiarch/memcpy.S: Check Fast_Unaligned_Load,
	instead of Slow_BSF, and also check for Fast_Copy_Backward to
	enable __memcpy_ssse3_back.

(cherry picked from commit 14a1d7cc)