test: match gccgo error messages

fixedbugs/issue4510.dir/f2.go:7: error: 'fmt' defined as both imported name and global name
f1.go:7: note: 'fmt' imported here

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/41530044

test/fixedbugs/issue4510.dir/f1.go

@@ -4,6 +4,6 @@
 
 package p
 
-import "fmt" // ERROR "fmt redeclared"
+import "fmt" // ERROR "fmt redeclared|imported"
 
 var _ = fmt.Printf