Auto merge of #77380 - fusion-engineering-forks:unbox-the-mutex, r=dtolnay

Unbox mutexes and condvars on some platforms

Both mutexes and condition variables contained a Box containing the actual os-specific object. This was done because moving these objects may cause undefined behaviour on some platforms.

However, this is not needed on Windows[1], Wasm[2], cloudabi[2], and 'unsupported'[3], were the box was only needlessly making them less efficient.

This change gets rid of the box on those platforms.

On those platforms, `Condvar` can no longer verify it is only used with one `Mutex`, as mutexes no longer have a stable address. This was addressed and considered acceptable in #76932.

[1]\: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
[2]\: These are just a single atomic integer together with futex wait/wake calls/instructions.
[3]\: The `unsupported` platform doesn't support multiple threads at all.

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