Add appengine and noasm build tags.

The general suggestion to use a noasm tag is by Klaus Post at
https://groups.google.com/d/msg/golang-dev/CeKX81B3WdQ/2mq-eY0VAgAJ

decode_amd64.go

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !appengine
 // +build gc
+// +build !noasm
 
 package snappy
 

decode_amd64.s

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !appengine
 // +build gc
+// +build !noasm
 
 #include "textflag.h"
 

decode_other.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 !gc
+// +build !amd64 appengine !gc noasm
 
 package snappy