Add Futex Constants FUTEX_WAITERS, FUTEX_OWNER_DIED and FUTEX_TID_MASK to Linux

[danielschemmel]

This PR adds three constants from linux/futex.h: FUTEX_WAITERS, FUTEX_OWNER_DIED and FUTEX_TID_MASK. These are related to how the kernel indicates certain events to the holders of priority-inheritance futexes (see man 2 futex).

I have intentionally typed those as u32, rather than ::c_int, as they are applied to the futex word (the target of the parameter uint32_t* uaddr).

[rustbot]

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[tgross35]

Can you target main? We will cherry-pick to stable.

@rustbot label +stable-nominated

[tgross35]

@rustbot author

[bors]

☔ The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

Thanks!

[danielschemmel]

@tgross35 Thank you for the review. The PR should now target main.