cmd/gc: mark wrapper methods for unnamed types as DUPOK.

Unnamed types like structs with embedded fields can have methods.
These methods are generated on-the-fly by the compiler and
it may happen for identical types in different packages.
The linker must accept these multiple definitions.

Fixes #4590.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/7030051

test/fixedbugs/issue4590.dir/pkg1.go

@@ -0,0 +1,26 @@
+// Copyright 2012 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 pkg1
+
+type A interface {
+	Write() error
+}
+
+type B interface {
+	Hello()
+	world()
+}
+
+type C struct{}
+
+func (c C) Write() error { return nil }
+
+var T = struct{ A }{nil}
+var U = struct{ B }{nil}
+var V A = struct{ *C }{nil}
+var W = interface {
+	Write() error
+	Hello()
+}(nil)