[libc++] Improve performance of std::atomic_flag on Windows (#163524) On Windows 8 and above, the WaitOnAddress, WakeByAddressSingle and WakeByAddressAll functions allow efficient implementation of the C++20 wait and notify features of std::atomic_flag. These Windows functions have never been made use of in libc++, leading to very poor performance of these features on Windows platforms, as they are implemented using a spin loop with backoff, rather than using any OS thread signalling whatsoever. This change implements the use of these OS functions where available, falling back to the original implementation on Windows versions prior to 8. Relevant API docs from Microsoft: https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitonaddress https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-wakebyaddresssingle https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-wakebyaddressall Fixes #127221
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