test: match gccgo error messages for blank1.go

blank1.go:10:9: error: invalid package name _
blank1.go:17:2: error: cannot use _ as value
blank1.go:18:7: error: cannot use _ as value
blank1.go:20:8: error: invalid use of ‘_’

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/14088044

test/blank1.go

@@ -21,7 +21,7 @@ func main() {
 	_()	// ERROR "cannot use _ as value"
 	x := _+1	// ERROR "cannot use _ as value"
 	_ = x
-	_ = t._ // ERROR "cannot refer to blank field"
+	_ = t._ // ERROR "cannot refer to blank field|invalid use of"
 
       var v1, v2 T
       _ = v1 == v2 // ERROR "cannot be compared|non-comparable"