Rollup merge of #160848 - joboet:opaque_c_alias, r=nia-e

std: avoid aliasing violations when wrapping opaque C types

Fixes rust-lang/rust#160815 (and some other instances of the same problem)

See the new documentation of `COpaque` for a detailed description of the kinds of issues solved by this. In short: creating mutable references to the opaque types from pthread is unsound since some platforms (at least AIX) store an intrinsically list of these types and the creation of the mutable reference (e.g. in the drop glue) invalidates the other pointers to the type.

This doesn't just apply to the internal pthread `Condvar` and `Mutex` abstraction as described in the issue, but also to all other opaque types – nobody is promising us that these are not internally aliased. Hence this PR adds an internal `COpaque` helper type which uses a combination of `UnsafePinned` and `MaybeUninit` to relax all relevant requirements added by Rust's operational semantics.

CC @RalfJung I'd love to hear your opinion on this
r? libs