Mark _wordcopy_XXX functions hidden

Since _wordcopy_XXX functions are only used internally in ld.so and
libc.so, they can be made hidden.

	[BZ #19122]
	* sysdeps/generic/memcopy.h (_wordcopy_fwd_aligned): Add
	attribute_hidden.
	(_wordcopy_fwd_dest_aligned): Likewise.
	(_wordcopy_bwd_aligned): Likewise.
	(_wordcopy_bwd_dest_aligned): Likewise.