test: add gcc6798, valid code that caused gccgo to crash

Change-Id: I288bd3091ea81db7b616747cbec8958a31d98b7e
Reviewed-on: https://go-review.googlesource.com/16532Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/gcc67968.dir/a.go

+// Copyright 2015 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 T int
+
+func (a *T) Foo() [1]string {
+	var r [1]string
+	return r
+}

test/fixedbugs/gcc67968.dir/b.go

+// Copyright 2015 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 b
+
+import "./a"
+
+func F() (interface{}) {
+     var v *a.T
+     return v.Foo()
+}

test/fixedbugs/gcc67968.go

+// compiledir
+
+// Copyright 2015 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.
+
+// https://gcc.gnu.org/PR67968
+
+// gccgo compiler crash building the equality and hash functions for a
+// type when a return statement requires a conversion to interface
+// type of a call of function defined in a different package that
+// returns an unnamed type.
+
+package ignored