all: support go get golang.org/x/mobile/... on Windows

Fixes golang/go#12212
Workaround for golang/go#12261 until golang/go#9306 is fixed.

Change-Id: I51c1bcfc92c1553fe2132586a0234b1c1af6aeb1
Reviewed-on: https://go-review.googlesource.com/13745
Reviewed-by: David Crawshaw <crawshaw@golang.org>

example/basic/main_x.go

@@ -0,0 +1,10 @@
+// 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.
+
+// +build !darwin,!linux
+
+package main
+
+func main() {
+}