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

src/trusted/service_runtime/linux/thread_suspension.c

Index: src/trusted/service_runtime/linux/thread_suspension.c
diff --git a/src/trusted/service_runtime/linux/thread_suspension.c b/src/trusted/service_runtime/linux/thread_suspension.c
index 6f4dc6720d4fdea94acba6aee417bf33717e9434..de76b44e3de8e57a90c47c37d367a518fd319bae 100644
--- a/src/trusted/service_runtime/linux/thread_suspension.c
+++ b/src/trusted/service_runtime/linux/thread_suspension.c
@@ -80,8 +80,7 @@ void NaClAppThreadSetSuspendState(struct NaClAppThread *natp,
 }
 
 static void HandleSuspendSignal(struct NaClSignalContext *regs) {
-  uint32_t tls_idx = NaClTlsGetIdx();
-  struct NaClAppThread *natp = NaClAppThreadGetFromIndex(tls_idx);
+  struct NaClAppThread *natp = NaClTlsGetCurrentThread();
   struct NaClSignalContext *suspended_registers =
       &natp->suspended_registers->context;