Auto merge of #107329 - joboet:optimize_lazylock, r=m-ou-se

Optimize `LazyLock` size

The initialization function was unnecessarily stored separately from the data to be initialized. Since both cannot exist at the same time, a `union` can be used, with the `Once` acting as discriminant. This unfortunately requires some extra methods on `Once` so that `Drop` can be implemented correctly and efficiently.

`@rustbot` label +T-libs +A-atomic