Rollup merge of #96040 - m-ou-se:futex-u32, r=Amanieu

Use u32 instead of i32 for futexes.

This changes futexes from i32 to u32. The [Linux man page](https://man7.org/linux/man-pages/man2/futex.2.html) uses `uint32_t` for them, so I'm not sure why I used i32 for them. Maybe because I first used them for thread parkers, where I used -1, 0, and 1 as the states.

(Wasm's `memory.atomic.wait32` does use `i32`, because wasm doesn't support `u32`.)

It doesn't matter much, but using the unsigned type probably results in fewer surprises when shifting bits around or using comparison operators.

r? ```@Amanieu```

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