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

A common use of memmove() can be handled by memcpy(). Also memcpy()
includes an optimization 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 also memcpy() if the source don't overlap
the destination (src + count <= dest).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>