[dev.power64] cmd/cc: round argsize to multiples of wordsize on power64

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/121640043
[dev.power64] cmd/9g: use loops to do memmove in sgen()

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/128130044
[dev.power64] liblink: rewrite MOVD $con(Ra), Rb into ADD $con, Ra, Rb; fix stack split prologue

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/129050043
[dev.power64] runtime: switch back to old initial stack size, and disable debugging messages for rewindmorestack

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/129850044