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

Author: joboet

tests/pass-dep/concurrency/tls_pthread_drop_order.rs

@@ -1,9 +1,9 @@
 //@ignore-target-windows: No pthreads on Windows
 //! Test that pthread_key destructors are run in the right order.
 //! Note that these are *not* used by actual `thread_local!` on Linux! Those use
-//! `thread_local_dtor::register_dtor` from the stdlib instead. In Miri this hits the fallback path
-//! in `register_dtor_fallback`, which uses a *single* pthread_key to manage a thread-local list of
-//! dtors to call.
+//! `destructors::register` from the stdlib instead. In Miri this ends up hitting
+//! the fallback path in `guard::key::enable`, which uses a *single* pthread_key
+//! to manage a thread-local list of dtors to call.
 
 use std::mem;
 use std::ptr;