string: Use memcpy() within memmove() when we can

A common use of memmove() can be handled by memcpy(). Also memcpy()
includes an optimisation for large sizes: it copies a word at a time. So
we can get a speed-up by calling memcpy() to handle our move in this case.

Update memmove() to call memcpy() if the destination is before the source.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>