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

gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,dsriram/golang