poly1305, chacha20poly1305: fix build for Go 1.6

Fixes golang/go#17424

Change-Id: I49d6e475c173da6a31542931d555ab87cc45a1c6
Reviewed-on: https://go-review.googlesource.com/30971
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
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
+// +build amd64,go1.7
 
 package chacha20poly1305
 

chacha20poly1305/chacha20poly1305_amd64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.7
+
 #include "textflag.h"
 // General register allocation
 #define oup DI

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
+// +build !amd64 !go1.7
 
 package chacha20poly1305
 

poly1305/sum_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,!gccgo,!appengine
+// +build amd64,!gccgo,!appengine,go1.7
 
 package poly1305
 

poly1305/sum_amd64.s

@@ -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,!gccgo,!appengine
+// +build amd64,!gccgo,!appengine,go1.7
 
 #include "textflag.h"
 

poly1305/sum_ref.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,!arm gccgo appengine
+// +build !amd64,!arm gccgo appengine !go1.7
 
 package poly1305