Implement unwind safety for Condvar on all platforms
Closes#118009
This commit adds unwind safety consistency to Condvar. Previously, only select platforms implemented unwind safety through auto traits. Known by this committer: On Linux, `Condvar` implemented `UnwindSafe` but on Mac and Windows, it did not. This change changes the implementation from auto to explicit.
In #118009, it was suggested that the platform differences were a bug and that a simple PR could address this. In trying to determine the best information to put in the `#[stable]` attribute, it [was suggested](https://github.com/rust-lang/rust/issues/121690#issuecomment-1968298470) I copy the stability information from the previous unwind safety implementations.
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