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

1 files changed, 0 insertions, 4 deletions

diff --git a/library/std/src/sys/pal/wasm/mod.rs b/library/std/src/sys/pal/wasm/mod.rs
index 75dd10826cc..4c34859e918 100644
--- a/library/std/src/sys/pal/wasm/mod.rs
+++ b/library/std/src/sys/pal/wasm/mod.rs
@@ -34,10 +34,6 @@ pub mod pipe;
 pub mod process;
 #[path = "../unsupported/stdio.rs"]
 pub mod stdio;
-#[path = "../unsupported/thread_local_dtor.rs"]
-pub mod thread_local_dtor;
-#[path = "../unsupported/thread_local_key.rs"]
-pub mod thread_local_key;
 #[path = "../unsupported/time.rs"]
 pub mod time;