test: match gccgo error messages for bug016.go.

bug016.go:11:8: error: negative shift count

R=rsc, gri
CC=golang-dev
https://golang.org/cl/4312055

test/fixedbugs/bug016.go

@@ -8,7 +8,7 @@ package main
 
 func main() {
 	var i int = 100
-	i = i << -3 // ERROR "overflows"
+	i = i << -3 // ERROR "overflows|negative"
 }
 
 /*