[dev.power64] runtime: fix usleep not to smash saved LR
[dev.power64] runtime: fix usleep not to smash saved LR

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/130760043

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