[dev.power64] cmd/gc: handle thechar=='9', disable nilopt() for now

LGTM=rsc
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/121630043
[dev.power64] liblink: support stack split, long conditional branches

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/123300043
[dev.power64] cmd/9l: remove MAXIO (fix build)

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/124270043
[dev.power64] cmd/9c: fix endian issue in integer to floating point conversion

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/129850043