Auto merge of #110861 - m-ou-se:thread-local-restructure, r=workingjubilee

Restructure and rename std thread_local internals to make it less of a maze

Every time I try to work on std's thread local internals, it feels like I'm trying to navigate a confusing maze made of macros, deeply nested modules, and types with multiple names/aliases. Time to clean it up a bit.

This PR:

- Exports `Key` with its own name (`Key`), instead of `__LocalKeyInner`
- Uses `pub macro` to put `__thread_local_inner` into a (unstable, hidden) module, removing `#[macro_export]`, removing it from the crate root.
- Removes the `__` from `__thread_local_inner`.
- Removes a few unnecessary `allow_internal_unstable` features from the macros
- Removes the `libstd_thread_internals` feature. (Merged with `thread_local_internals`.)
    - And removes it from the unstable book
- Gets rid of the deeply nested modules for the `Key` definitions (`mod fast` / `mod os` / `mod statik`).
- Turns a `#[cfg]` mess into a single `cfg_if`, now that there's no `#[macro_export]` anymore that breaks with `cfg_if`.
- Simplifies the `cfg_if` conditions to not repeat the conditions.
- Removes useless `normalize-stderr-test`, which were left over from when the `Key` types had different names on different platforms.
- Removes a seemingly unnecessary `realstd` re-export on `cfg(test)`.

This PR changes nothing about the thread local implementation. That's for a later PR. (Which should hopefully be easier once all this stuff is a bit cleaned up.)

library/std/src/thread/mod.rs

@@ -204,9 +204,12 @@ pub use self::local::{AccessError, LocalKey};
 // by the elf linker. "static" is for single-threaded platforms where a global
 // static is sufficient.
 
+// Implementation details used by the thread_local!{} macro.
 #[doc(hidden)]
-#[unstable(feature = "libstd_thread_internals", issue = "none")]
-pub use crate::sys::common::thread_local::Key as __LocalKeyInner;
+#[unstable(feature = "thread_local_internals", issue = "none")]
+pub mod local_impl {
+    pub use crate::sys::common::thread_local::{thread_local_inner, Key};
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 // Builder