[dev.garbage] cmd/gc: emit pointer write barriers
[dev.garbage] cmd/gc: emit pointer write barriers

This got lost in the change that added the writebarrierfat variants.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://codereview.appspot.com/165510043

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