switch everything to the now-standardized thread_local

We're only using compilers new enough to support it now, so just use it
without a compatibility layer in between.

Change-Id: I97a24ed344e3d4ccde0b04af9c44995cf417a635
diff --git a/aos/common/libc/aos_strsignal.cc b/aos/common/libc/aos_strsignal.cc
index 5fae327..2321a07 100644
--- a/aos/common/libc/aos_strsignal.cc
+++ b/aos/common/libc/aos_strsignal.cc
@@ -2,11 +2,10 @@
 
 #include <signal.h>
 
-#include "aos/linux_code/thread_local.h"
 #include "aos/common/logging/logging.h"
 
 const char *aos_strsignal(int signal) {
-  static AOS_THREAD_LOCAL char buffer[512];
+  static thread_local char buffer[512];
 
   if (signal >= SIGRTMIN && signal <= SIGRTMAX) {
     CHECK(snprintf(buffer, sizeof(buffer), "Real-time signal %d",