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

pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang