[libcxx] Fix _LIBCPP_HAS_THREAD_API_EXTERNAL build

[miyuki]

Summary

The definition of __libcpp_timed_backoff_policy and the declaration of
__libcpp_thread_poll_with_backoff must not be guarded by

#if !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)

because the definitions of __libcpp_timed_backoff_policy::operator()
and __libcpp_thread_poll_with_backoff aren't guarded by this macro
(and this is correct because these two functions are implemented in
terms of other libc++ functions and don't interact with the host
threading library).

libcxx/include/__threading_support

 int __libcpp_thread_detach(__libcpp_thread_t *__t);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 void __libcpp_thread_yield();
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 void __libcpp_thread_sleep_for(const chrono::nanoseconds& __ns);
 
-struct __libcpp_timed_backoff_policy {
-  _LIBCPP_THREAD_ABI_VISIBILITY
-  bool operator()(chrono::nanoseconds __elapsed) const;
-};
-
-template<class _Fn, class _BFn>
-_LIBCPP_INLINE_VISIBILITY
-bool __libcpp_thread_poll_with_backoff(
-  _Fn && __f, _BFn && __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero());
-
 // Thread local storage
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_tls_create(__libcpp_tls_key* __key,
                         void(_LIBCPP_TLS_DESTRUCTOR_CC* __at_exit)(void*));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 void *__libcpp_tls_get(__libcpp_tls_key __key);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
 #endif // !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 
 #if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
      defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL))
 
+struct __libcpp_timed_backoff_policy {
+  _LIBCPP_THREAD_ABI_VISIBILITY
+  bool operator()(chrono::nanoseconds __elapsed) const;
+};
+
+template<class _Fn, class _BFn>
+_LIBCPP_INLINE_VISIBILITY
+bool __libcpp_thread_poll_with_backoff(
+  _Fn && __f, _BFn && __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero());
+
 namespace __thread_detail {
 
 inline __libcpp_timespec_t __convert_to_timespec(const chrono::nanoseconds& __ns)
 {
   using namespace chrono;
   seconds __s = duration_cast<seconds>(__ns);
   __libcpp_timespec_t __ts;
   typedef decltype(__ts.tv_sec) __ts_sec;