[dev.power64] cmd/9g: introduce ginscon2 for CMP/CMPU, use ginscon to ADD constants

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125170043
[dev.power64] runtime: fix morestack

Must not save LR.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/129040044