std: replace `LazyBox` with `OnceBox`
This PR replaces the `LazyBox` wrapper used to allocate the pthread primitives with `OnceBox`, which has a more familiar API mirroring that of `OnceLock`. This cleans up the code in preparation for larger changes like #128184 (from which this PR was split) and allows some neat optimizations, like avoid an acquire-load of the allocation pointer in `Mutex::unlock`, where the initialization of the allocation must have already been observed.
Additionally, I've gotten rid of the TEEOS `Condvar` code, it's just a duplicate of the pthread one anyway and I didn't want to repeat myself.
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