go/gccgoimporter: fix tests for Plan9

This is a backport of https://go-review.googlesource.com/#/c/11194/.

Change-Id: I2f8a8efec77a29258c85f5a1b11cd9deb81853d0
Reviewed-on: https://go-review.googlesource.com/11210
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/gccgoimporter/importer.go

@@ -91,6 +91,12 @@ 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[:]) {