misc/cgo/test/backdoor: add gccgo version of backdoor function

For the gc compiler the Go function Issue7695 is defined in
runtime.c, but there is no way to do that for gccgo, because
there is no way to get the correct pkgpath.  The test is not
important for gccgo in any case.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/93870044

misc/cgo/test/backdoor/backdoor_gccgo.go

+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is the gccgo version of the stub in runtime.c.
+
+// +build gccgo
+
+package backdoor
+
+func Issue7695(x1, x2, x3, x4, x5, x6, x7, x8 uintptr) {}