go/internal/gccgoimporter: enable tests on Plan9

Work-around issue #11265 and re-enable tests for Plan9.

Change-Id: I3aabb674a149b8eb936f948dd4cda5fd81454646
Reviewed-on: https://go-review.googlesource.com/11194
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/internal/gccgoimporter/gccgoinstallation_test.go

@@ -2,10 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Exclude plan9 for now due to test failure with TestGoxImporter.
-// TODO(gri) eliminate this build tag
-// +build !plan9
-
 package gccgoimporter
 
 import (

src/go/internal/gccgoimporter/importer.go

@@ -88,6 +88,12 @@ func openExportFile(fpath string) (reader io.ReadSeeker, closer io.Closer, err e
 	if err != nil {
 		return
 	}
+	// reset to offset 0 - needed on Plan 9 (see issue #11265)
+	// TODO: remove once issue #11265 has been resolved.
+	_, err = f.Seek(0, 0)
+	if err != nil {
+		return
+	}
 
 	var elfreader io.ReaderAt
 	switch string(magic[:]) {

src/go/internal/gccgoimporter/importer_test.go

@@ -2,10 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Exclude plan9 for now due to test failure with TestGoxImporter.
-// TODO(gri) eliminate this build tag
-// +build !plan9
-
 package gccgoimporter
 
 import (