Match gccgo error messages.

ddd1.go:16:10: error: argument 1 has incompatible type
ddd1.go:17:10: error: argument 1 has incompatible type
ddd1.go:15:10: error: floating point constant truncated to integer

R=rsc
CC=golang-dev
https://golang.org/cl/204048

test/ddd1.go

@@ -13,8 +13,8 @@ var (
 	_ = sum()
 	_ = sum(1.0, 2.0)
 	_ = sum(1.5)      // ERROR "integer"
-	_ = sum("hello")  // ERROR "convert"
-	_ = sum([]int{1}) // ERROR "slice literal.*as type int"
+	_ = sum("hello")  // ERROR "convert|incompatible"
+	_ = sum([]int{1}) // ERROR "slice literal.*as type int|incompatible"
 )
 
 type T []T