restructure thread_local! for better codegen (especially on macos)

Author: mtak-

src/libstd/sys/redox/fast_thread_local.rs

@@ -1,4 +1,4 @@
 #![cfg(target_thread_local)]
 #![unstable(feature = "thread_local_internals", issue = "0")]
 
-pub use crate::sys_common::thread_local::register_dtor_fallback as register_dtor;
+pub use crate::sys_common::thread_local::register_dtor_fallback as register_dtor;