Use size_t for loop counter to avoid sign comparison warning

src/nonsfi/linux/irt_signal_handling.c

 /*
  * Copyright (c) 2015 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 <errno.h>
 #include <pthread.h>
 #include <signal.h>
 #include <stdint.h>
@@ skipping 459 matching lines @@
    */
   memset(&mask, 0, sizeof(mask));
 
   /*
    * Hack to be able to reuse sigset_t utilities from newlib for the
    * first lower 4 bytes of the signal, works because we are all
    * little endians.
    */
   sigset_t *maskptr = (sigset_t *) &mask;
   sigemptyset(maskptr);
-  for (int a = 0; a < NACL_ARRAY_SIZE(kSignals); a++) {
+  for (size_t a = 0; a < NACL_ARRAY_SIZE(kSignals); a++) {
     if (sigaddset(maskptr, kSignals[a]) != 0)
       abort();
   }
   if (linux_sigprocmask(LINUX_SIG_UNBLOCK, &mask, NULL) != 0)
     abort();
 
   return 0;
 }
 
 int nacl_exception_set_stack(void *p, size_t s) {
@@ skipping 13 matching lines @@
 
 int nacl_async_signal_send_async_signal(nacl_irt_tid_t tid) {
   if (!g_signal_handler_initialized)
     return ESRCH;
   if (tid == 0)
     tid = g_main_tid;
   if (linux_tgkill(g_tgid, tid, LINUX_SIGUSR1) == -1)
     return errno;
   return 0;
 }