test: add test for gccgo-specific issue 8079

LGTM=gri
R=gri
CC=golang-codereviews, iant
https://golang.org/cl/159850044

test/fixedbugs/issue8079.go

+// compile
+
+// 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.
+
+// Issue 8079: gccgo crashes when compiling interface with blank type name.
+
+package p
+
+type _ interface{}