test: add test that gccgo failed to compile

For #38125

Change-Id: Id6ef10d74f0f9dbad2851531e0fe019cd145cf7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/251168
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/issue38125.go

@@ -0,0 +1,22 @@
+// compile
+
+// Copyright 2020 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 mishandled embedded methods of type aliases.
+
+package p
+
+type I int
+
+func (I) M() {}
+
+type T = struct {
+	I
+}
+
+func F() {
+	_ = T.M
+	_ = struct { I }.M
+}