test: add test that caused gccgo incorrect compilation

Updates #21770

Change-Id: Ic31c3bdae30797f406f25c737b83bbe2de1ed1da
Reviewed-on: https://go-review.googlesource.com/62331Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue21770.go

+// errorcheck
+
+// Copyright 2017 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 21770: gccgo incorrectly accepts "p.f = 0" where p is **struct
+
+package p
+
+type PP **struct{ f int }
+
+func f() {
+	// anonymous type
+	var p **struct{ f int }
+	p.f = 0 // ERROR "field"
+	// named type
+	var p2 PP
+	p2.f = 0 // ERROR "field"
+}