container/intsets: build correctly on App Engine

App Engine is amd64 but it doesn't support uploading assembly files.
Use the explicit build tag so that it selects the generic implementation
on App Engine.
This is required to deploy golang.org.

Change-Id: I7374c91961c53d59f6fdcc9ac98b8a9cec755b2c
Reviewed-on: https://go-review.googlesource.com/15246
Reviewed-by: Andrew Gerrand <adg@golang.org>

container/intsets/popcnt_amd64.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build amd64
+// +build amd64,!appengine
 
 package intsets
 

container/intsets/popcnt_generic.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64
+// +build !amd64 appengine
 
 package intsets