cmd/gc: make bisonerrors compatible with GNU Bison 3.0
cmd/gc: make bisonerrors compatible with GNU Bison 3.0

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/11990043

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