runtime: avoid r9/r10 during memmove
runtime: avoid r9/r10 during memmove

Fixes issue 3718.

Requires CL 6300043.

R=rsc, minux.ma, extraterrestrial.neighbour
CC=golang-dev
http://codereview.appspot.com/6305100

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang