Fix observable intermediate state in thread::add_spawn_hook

[maxdexh]

Fixes #159923.

Ensures the intermediate state of there being no hooks in add_spawn_hooks is not observable by allocating the new head node before taking the current hook list.

Also added a note to the docs of add_spawn_hook that hooks are not guaranteed to run and cannot be relied upon for soundness, as there are multiple ways to prevent all/some hooks from running.

[rustbot]

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[maxdexh]

• Owners of files modified in this PR: compiler

This doesn't touch the compiler?

[folkertdev]

I guess all of the compiler is compiler? anyhow, I think you can also explicitly

r? libs

[maxdexh]

Thanks ^^
It's probably because I had to add the test in ui/std (at least it didn't work anywhere else, idk if I did something wrong), since it uses a custom global allocator.

[Mark-Simulacrum]

@bors r+ rollup

[rust-bors]

📌 Commit f80e5c1 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures