[dev.power64] cmd/9g: add missing break

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125260043

Committer: Russ Cox <rsc@golang.org>
[dev.power64] cmd/9g: fix floating-point comparison for NaN

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/127300043
[dev.power64] runtime: fix C prototype for cmallocgc

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/122520043