std: allow Condvar to be used with multiple Mutexes

[joboet]

The documentation for Condvar::wait currently states that

This function may panic! if it is used with more than one mutex over time.

This is however only true on UNIX platforms that still use the pthread Condvar implementation (namely macOS, some BSDs like NetBSDs and some very niche platforms like VxWorks), making it likely that users will not be aware of this caveat.

Hence in this PR, I've modified the pthread-based implementation to allow multi-mutex-use by adding an inner mutex that bridges the gap between the unlock operation on the user-specified mutex and the actual condvar wait.

Note that this closes the door on making the futex-based notify_all() a requeuing operation, as the waiters might not be sharing the same mutex. At the same time, the implementation change here makes it easier to switch the Mutex implementation independent of the Condvar one, which would e.g. simplify switching it to employ os_unfair_lock on macOS.

@rustbot label +I-libs-api-nominated

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.