[dev.power64] cmd/9g: clear p->reg for ATEXT

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125150043
[dev.power64] runtime: disable GC and use a larger default stack

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/128120043