runtime: homogenize panic strings on Plan 9
runtime: homogenize panic strings on Plan 9

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/61410046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang