Remove nacl_user[] array lookup from syscall code path on x86-64 and MIPS

tests/thread_capture/arch/x86_64/thread_capture_test_injection.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 "native_client/tests/thread_capture/thread_capture_test_injection.h"
 
 #include "native_client/src/include/portability.h"
 #include "native_client/src/trusted/service_runtime/include/bits/nacl_syscalls.h"
 #include "native_client/src/trusted/service_runtime/nacl_app_thread.h"
 #include "native_client/src/trusted/service_runtime/nacl_copy.h"
 #include "native_client/src/trusted/service_runtime/nacl_syscall_common.h"
 #include "native_client/src/trusted/service_runtime/nacl_tls.h"
 
 static int32_t TestSyscall(struct NaClAppThread *natp) {
   g_nacl_syscall_thread_capture_fault_addr =
       (uintptr_t) &NaClSyscallThreadCaptureFault;
 
   NaClCopyDropLock(natp->nap);
 
-  NaClTlsSetIdx(0);
+  NaClTlsSetCurrentThread(NULL);
 
   return 0;
 }
 
 void NaClInjectThreadCaptureSyscall(struct NaClApp *nap) {
   UNREFERENCED_PARAMETER(nap);
 
   NaClAddSyscall(NACL_sys_test_syscall_1, TestSyscall);
 }