It doesn't actually matter unless there's a preemption in the middle of a memcpy and something relies on 16-byte chunks to be atomically copied, but that seems extremely unlikely and the emu can't even be preempted inside a HLE:d function anyway.

So, it really doesn't matter in any way. Just add a comment.

If you really want to copy 16-bit quantities, use SSE intrinsics on x86 and NEON on ARM but there's not really any point.