Recognize gccgo error messages:

interface5.go:15:5: error: incompatible types in assignment
interface5.go:16:5: error: incompatible types in assignment

(Yes, these could be better).

R=rsc
DELTA=2  (0 added, 0 deleted, 2 changed)
OCL=22821
CL=22826

test/interface5.go

@@ -16,6 +16,6 @@ func main() {
 	// neither of these can work,
 	// because i has an extra method
 	// that t does not, so i cannot contain a t.
-	i = t;	// ERROR "missing"
-	t = i;	// ERROR "missing"
+	i = t;	// ERROR "missing|incompatible"
+	t = i;	// ERROR "missing|incompatible"
 }