poly1305: enable assembly for ARM in Go 1.6.

5f31782cfb2b6373211f8f9fbf31283fa234b570 added build constraints to
disable assembly for Go 1.6 but didn't add the needed tags to the ARM
files. Also, it's not clear that was needed as the error given in
golang/go#17424 only complains about the chacha20poly1305 package.

This change reenables the assembly for Go 1.6 in the poly1305 package.
Tested with 1.6.3 and 1.5.4.

Fixes golang/go#17512.

Change-Id: I81b41f8810437ea327b415542402cd8ff5c8a390
Reviewed-on: https://go-review.googlesource.com/31492
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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