test: Match gccgo error messages.

gccgo does not handle 'new' specially here.

varerr.go:10:6: error: reference to undefined name ‘asdf’
varerr.go:12:6: error: invalid left hand side of assignment

R=rsc
CC=golang-dev
https://golang.org/cl/2139045

test/varerr.go

@@ -7,8 +7,8 @@
 package main
 
 func main() {
-	_ = asdf	// ERROR "undefined: asdf"
+	_ = asdf	// ERROR "undefined.*asdf"
 
-	new = 1	// ERROR "use of builtin new not in function call"
+	new = 1	// ERROR "use of builtin new not in function call|invalid left hand side"
 }