Revert thread_local stuff

Unfortunatly accessing thread local variable
is much slower than object data (see previous
patch log msg), so we have to revert to old code
to avoid speed regression.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/thread.h

@@ -119,7 +119,7 @@ public:
   bool use_sleeping_threads() const { return useSleepingThreads; }
   int min_split_depth() const { return minimumSplitDepth; }
   int size() const { return (int)threads.size(); }
-  Thread* main_thread() const { return threads[0]; }
+  Thread* main_thread() { return threads[0]; }
 
   void wake_up() const;
   void sleep() const;
@@ -146,6 +146,5 @@ private:
 };
 
 extern ThreadsManager Threads;
-extern THREAD_LOCAL Thread* this_thread;
 
 #endif // !defined(THREAD_H_INCLUDED)