Auto merge of #98457 - japaric:gh98378, r=m-ou-se

Auto merge of #98457 - japaric:gh98378, r=m-ou-se

make Condvar, Mutex, RwLock const constructors work with the `unsupported` impl

applying this patch locally to the `rust-src` component fixes #98378

however, the solution seems wrong to me because PR #97791 didn't add any `rustc_const_stable` attribute to underlying implementations like `std::sys::unix::futex`, so I must be missing something about how const-stability is checked ... maybe the `restricted_std` feature (gate?) has an effect?

fixes #98378
fixes #98293 (probably)