test: add fixedbugs/bug506 for gccgo

Building with gccgo failed with an undefined symbol error from an
unnecessary hash function.

Updates #19773

Change-Id: Ic78bf1b086ff5ee26d464089c0e14987d3fe8b02
Reviewed-on: https://go-review.googlesource.com/c/130956
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/bug506.dir/a.go

+// Copyright 2018 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.
+
+package a
+
+type internal struct {
+	f1 string
+	f2 float64
+}
+
+type S struct {
+	F struct {
+		I internal
+	}
+}

test/fixedbugs/bug506.dir/main.go

+// Copyright 2018 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.
+
+package main
+
+import (
+	"fmt"
+
+	"./a"
+)
+
+var v = a.S{}
+
+func main() {
+	want := "{{ 0}}"
+	if got := fmt.Sprint(v.F); got != want {
+		panic(got)
+	}
+}

test/fixedbugs/bug506.go

+// rundir
+
+// Copyright 2018 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.
+
+// Gccgo caused an undefined symbol reference building hash functions
+// for an imported struct with unexported fields.
+
+package ignored