test: Recognize gccgo error messages.

bug278.go:18: error: invalid left hand side of assignment
bug278.go:19: error: array is not addressable
bug278.go:21: error: invalid left hand side of assignment
bug278.go:22: error: invalid left hand side of assignment

R=rsc
CC=golang-dev
https://golang.org/cl/2122042

test/fixedbugs/bug278.go

@@ -15,9 +15,9 @@ func f() [10]int {
 var m map[int][10]int
 
 func main() {
-	f()[1] = 2	// ERROR "cannot"
-	f()[2:3][0] = 4	// ERROR "cannot"
+	f()[1] = 2	// ERROR "cannot|invalid"
+	f()[2:3][0] = 4	// ERROR "cannot|addressable"
 	var x = "abc"
-	x[2] = 3	// ERROR "cannot"
-	m[0][5] = 6  // ERROR "cannot"
+	x[2] = 3	// ERROR "cannot|invalid"
+	m[0][5] = 6  // ERROR "cannot|invalid"
 }