test: adjust gccgo expected import errors

Update two tests for the recently submitted gccgo change
https://golang.org/cl/14259.

Change-Id: Ib18bc87ea512074aa91fd4096d0874b72e2243e5
Reviewed-on: https://go-review.googlesource.com/15493Reviewed-by: Chris Manghane <cmang@golang.org>

test/fixedbugs/issue5957.dir/c.go

@@ -2,7 +2,7 @@ package p
 
 import (
 	"./a" // ERROR "imported and not used: \x22a\x22 as surprise|imported and not used: surprise"
-	"./b" // GC_ERROR "imported and not used: \x22b\x22 as surprise2|imported and not used: surprise2"
+	"./b" // ERROR "imported and not used: \x22b\x22 as surprise2|imported and not used: surprise2"
 	b "./b" // ERROR "imported and not used: \x22b\x22$|imported and not used: surprise2"
 	foo "math" // ERROR "imported and not used: \x22math\x22 as foo|imported and not used: math"
 	"fmt" // actually used

test/import4.dir/import4.go

@@ -18,7 +18,7 @@ import X "math"	// ERROR "imported and not used.*math"
 import . "bufio"	// ERROR "imported and not used.*bufio"
 
 // again, package without anything in it
-import "./empty"	// GC_ERROR "imported and not used.*empty"
-import Z "./empty"	// GC_ERROR "imported and not used.*empty"
+import "./empty"	// ERROR "imported and not used.*empty"
+import Z "./empty"	// ERROR "imported and not used.*empty"
 import . "./empty"	// ERROR "imported and not used.*empty"