[dev.power64] runtime: implemnt runtime.switchtoM and runtime.onM

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/129860043
[dev.power64] liblink: disable stack split for now

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/122460043