arm: Make sel build with clang's built-in assembler.

src/trusted/service_runtime/linux/arm/sel_segments.c

 /*
  * Copyright (c) 2012 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <stdint.h>
 #include "native_client/src/trusted/service_runtime/sel_util.h"
 
 uint32_t NaClGetStackPtr(void) {
   uint32_t sp;
 
-  asm("mov %0, %%sp" : "=r" (sp));
+  asm("mov %0, sp" : "=r" (sp));
 
   return sp;
 }