[dev.cc] runtime: convert dragonfly/amd64 port to Go

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://codereview.appspot.com/176750043
[dev.cc] runtime: fix linux build

TBR=austin
CC=golang-codereviews
https://codereview.appspot.com/176760044
[dev.cc] runtime: fix lfstack for amd64 addresses in top half of addr space

While we are here, add the linux/power64 version.

LGTM=austin
R=austin
CC=aram, dvyukov, golang-codereviews
https://codereview.appspot.com/177750043