Use thread_local instead of FOLLY_TLS in folly::observer

Summary:
`FOLLY_TLS` is explicitly disabled for 32-bit iOS. Perhaps because old iOS versions did not support `__thread`.

`thread_local` is standard C++11, appears to be supported by the iOS 32-bit build, and is also used in multiple places around folly already.

Differential Revision: D25287484

fbshipit-source-id: 2e81e275e0f1cba7bcf5239fe34d4b5eb8add6d0

folly/experimental/observer/detail/ObserverManager.cpp

@@ -29,8 +29,8 @@
 namespace folly {
 namespace observer_detail {
 
-FOLLY_TLS bool ObserverManager::inManagerThread_{false};
-FOLLY_TLS ObserverManager::DependencyRecorder::Dependencies*
+thread_local bool ObserverManager::inManagerThread_{false};
+thread_local ObserverManager::DependencyRecorder::Dependencies*
     ObserverManager::DependencyRecorder::currentDependencies_{nullptr};
 
 DEFINE_int32(

folly/experimental/observer/detail/ObserverManager.h

@@ -170,7 +170,7 @@ class ObserverManager {
     Dependencies dependencies_;
     Dependencies* previousDepedencies_;
 
-    static FOLLY_TLS Dependencies* currentDependencies_;
+    static thread_local Dependencies* currentDependencies_;
   };
 
  private:
@@ -195,7 +195,7 @@ class ObserverManager {
 
   static ObserverManager& getInstance();
   static std::shared_ptr<UpdatesManager> getUpdatesManager();
-  static FOLLY_TLS bool inManagerThread_;
+  static thread_local bool inManagerThread_;
 
   /**
    * Version mutex is used to make sure all updates are processed from the