test: match gccgo error messages for cmp6.go

cmp6.go:28:9: error: incompatible types in binary expression
cmp6.go:29:9: error: incompatible types in binary expression
cmp6.go:40:9: error: incompatible types in binary expression
cmp6.go:41:9: error: incompatible types in binary expression
cmp6.go:48:9: error: expected integer, floating, complex, string, pointer, boolean, interface, slice, map, channel, or function type

R=rsc, r2
CC=golang-dev
https://golang.org/cl/4281071

test/cmp6.go

@@ -25,8 +25,8 @@ func main() {
 	var c2 <-chan int
 	var c3 chan int
 	
-	use(c1 == c2)	// ERROR "invalid operation"
-	use(c2 == c1)	// ERROR "invalid operation"
+	use(c1 == c2)	// ERROR "invalid operation|incompatible"
+	use(c2 == c1)	// ERROR "invalid operation|incompatible"
 	use(c1 == c3)
 	use(c2 == c2)
 	use(c3 == c1)
@@ -37,13 +37,13 @@ func main() {
 	var p2 T2
 	var p3 *int
 	
-	use(p1 == p2)	// ERROR "invalid operation"
-	use(p2 == p1)	// ERROR "invalid operation"
+	use(p1 == p2)	// ERROR "invalid operation|incompatible"
+	use(p2 == p1)	// ERROR "invalid operation|incompatible"
 	use(p1 == p3)
 	use(p2 == p2)
 	use(p3 == p1)
 	use(p3 == p2)
 	
 	// Comparison of structs should have a good message
-	use(t3 == t3)	// ERROR "struct"
+	use(t3 == t3)	// ERROR "struct|expected"
 }