Optimize RETURN-MULTIPLE on x86-64.

Optimize RETURN-MULTIPLE on x86-64.

Replace REP MOVS with simple instructions.

RETURN-MULTIPLE is used to copy the multiple value down the stack.
The following code becomes around 5 times faster on a modern CPU,
passing more values is faster as well, but not by as much.

(defun m ()
  (declare (optimize speed (safety 0)))
  (values 1 2 3 4))

(defun b ()
  (declare (optimize speed (safety 0)))
  (let (*)
    (m)))