Do not wrap platform locks in sys_common

[joboet]

Now that Mutex and RwLock are const, StaticMutex and StaticRwLock are no longer necessary. Removing them results in a extra few allocations on platforms which require boxed locks (currently some UNIXes and SGX), but it makes the codebase quite a lot cleaner. With the lock improvements tracked in #93740, these allocations will become unnecessary.

The rest of the lock abstractions in sys_common can be removed by modifying the lock implementations in sys so that they are const and movable on all platforms. This entails a slight bit of code duplication (both SGX and UNIX need to wrap their locks in LazyBox, for now), but reduces the usage of unsafe quite considerably.

The Condvar check is merged into the pthread_condvar implementation, making sys_common::condvar redundant. The check is otherwise only performed on SGX and Hermit, which however do not rely on the check for soundness. I therefore removed it on these platforms, as it will be made redundant by the aforementioned lock improvements anyway.

Additionally, this PR makes the condition variable on Hermit movable by lazily initializing it in place instead of lazily allocating and initializing.

@rustbot label +T-libs, +T-libs-api
r? @m-ou-se

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[m-ou-se]

Always good to see a PR that removes more code than it adds. ^^

It's quite a large PR though, affecting a lot of different things, so it'd be good if you could split it up a bit. Then if this accidentally causes some issues later, it'll be easier to track down.

• It seems to me it should be easy to split off the changes that make all MovableThing = Thing on all platforms. (One PR for all platforms, or one PR each; whatever you prefer.) Then we can merge that first, making the remaining change less complicated.

• The removal of imp::requires_synchronized_create() is mostly unrelated to this change, and should be possible to do separately in an independent PR.

• The changes to stdout() don't only remove the Pin, but also move the lazy initialization of the buffer from the stdout() call to the first write() call. Is there any benefit in doing it that way? If so, it'd be good to put that in a separate PR.

[m-ou-se]

Why did this change? This seems unrelated to this PR.

[joboet]

I got a bit too carried away with the changes... 😉

This PR is now split in #100576, #100579, #100581 and #100583, the fifth part will be removing the sys_common wrappers, but I'll wait with that until the others are merged.