[dev.power64] liblink: rewrite MOVD $con(Ra), Rb into ADD $con, Ra, Rb; fix stack split prologue
[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

webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh