Migrate panic_unwind to use cfg_select!

[cuviper]

This follows #145489 with an additional place we can drop the cfg-if dependency.

[rustbot]

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[rustbot]

These commits modify the library/Cargo.lock file. Unintentional changes to library/Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.