runtime: fix arm build
runtime: fix arm build

TBR=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/140990043

bryanxu/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh