runtime: change concatstring panic to gothrow
runtime: change concatstring panic to gothrow

It was a throw originally; it was converted incorrectly.

LGTM=dave
R=khr, dave
CC=golang-codereviews
https://codereview.appspot.com/139000043

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