test: match gccgo error messages for bug330.go.

bug330.go:11:6: error: expected numeric type
bug330.go:12:6: error: expected numeric type

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5031050

test/fixedbugs/bug330.go

@@ -8,6 +8,6 @@ package main
 
 func main() {
 	x := ""
-	x = +"hello"  // ERROR "invalid operation.*string"
-	x = +x  // ERROR "invalid operation.*string"
+	x = +"hello"  // ERROR "invalid operation.*string|expected numeric"
+	x = +x  // ERROR "invalid operation.*string|expected numeric"
 }