go.crypto: various: fix appengine compatibility

Fixes golang/go#4102.

R=russross, minux.ma, rsc, agl
CC=golang-dev
https://golang.org/cl/6623053

salsa20/salsa/salsa2020_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 !appengine
+
 // This code was translated into a form compatible with 6a from the public
 // domain sources in SUPERCOP: http://bench.cr.yp.to/supercop.html
 

salsa20/salsa/salsa20_amd64.go

@@ -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 !appengine
+
 package salsa
 
 // This function is implemented in salsa2020_amd64.s.

salsa20/salsa/salsa20_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
+// +build !amd64 appengine
 
 package salsa