crypto/rc4: disable assembly implementation for nacl/arm.

It uses an unsupported addressing mode.

LGTM=iant, rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/106370043

src/pkg/crypto/rc4/rc4_arm.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 !nacl
+
 #include "../../../cmd/ld/textflag.h"
 
 // Registers

src/pkg/crypto/rc4/rc4_asm.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 amd64p32 arm 386
+// +build amd64 amd64p32 arm,!nacl 386
 
 package rc4
 

src/pkg/crypto/rc4/rc4_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,!amd64p32,!arm,!386
+// +build !amd64,!amd64p32,!arm,!386 arm,nacl
 
 package rc4