Mark implicitly inlined static locals with default visibility

Summary:
These functions contain static locals which need to be exported to
the global dynamic symbol table to get deduplicated at runtime.  Since
using `-fvisibility-inlines-hidden` demotes the visibility of these
static locals, explicitly tag them.

Reviewed By: yfeldblum

Differential Revision: D4139154

fbshipit-source-id: 08b81721c6c945ac5556472481ae771d0eeb5f88

folly/fibers/FiberManagerInternal.h

@@ -25,6 +25,7 @@
 #include <vector>
 
 #include <folly/AtomicIntrusiveLinkedList.h>
+#include <folly/CPortability.h>
 #include <folly/Executor.h>
 #include <folly/IntrusiveList.h>
 #include <folly/Likely.h>
@@ -253,7 +254,7 @@ class FiberManager : public ::folly::Executor {
   T& local();
 
   template <typename T>
-  static T& localThread();
+  FOLLY_EXPORT static T& localThread();
 
   /**
    * @return How many fiber objects (and stacks) has this manager allocated.