Auto merge of #126523 - joboet:the_great_big_tls_refactor, r=Mark-Simulacrum

std: refactor the TLS implementation

As discovered by Mara in #110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with #117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` modules to the `thread_local` module in `sys` and merged them into a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`.

Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones:
* the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code.
* I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that

Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps.

`@rustbot` label +A-thread-locals

library/std/src/sys_common/mod.rs

@@ -24,18 +24,9 @@ pub mod fs;
 pub mod io;
 pub mod lazy_box;
 pub mod process;
-pub mod thread_local_dtor;
 pub mod wstr;
 pub mod wtf8;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "windows")] {
-        pub use crate::sys::thread_local_key;
-    } else {
-        pub mod thread_local_key;
-    }
-}
-
 cfg_if::cfg_if! {
     if #[cfg(any(
         all(unix, not(target_os = "l4re")),