chacha20poly1305: fix build constraints

Similarly to https://go-review.googlesource.com/32311,
chacha20poly1305 needs additional build constraints
for non-standard toolchains.

Change-Id: I22816ef333c05450e9ab4debb3ce591518b6b84b
Reviewed-on: https://go-review.googlesource.com/32391
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

chacha20poly1305/chacha20poly1305_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,go1.7
+// +build go1.7,amd64,!gccgo,!appengine
 
 package chacha20poly1305
 

chacha20poly1305/chacha20poly1305_amd64.s

@@ -4,7 +4,7 @@
 
 // This file was originally from https://golang.org/cl/24717 by Vlad Krasnov of CloudFlare.
 
-// +build go1.7
+// +build go1.7,amd64,!gccgo,!appengine
 
 #include "textflag.h"
 // General register allocation

chacha20poly1305/chacha20poly1305_noasm.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 !go1.7
+// +build !amd64 !go1.7 gccgo appengine
 
 package chacha20poly1305