Add data_ptr method to Mutex and RwLock

[jplatte]

Implementation of #140368 / rust-lang/libs-team#531.

I tried to write a useful safety section about when it is safe to read or write through the returned pointers, but couldn't come up with something nice. Hoping this PR is still useful without that. I'm happy to add any doc strings other people come up with if needed before merge, of course.

Unresolved questions:

• Return a LockResult or not?
• Return *mut T like existing APIs (Cell::as_ptr / MaybeUninit::as[_mut]_ptr / Vec::as_ptr / ...) or be more precise and return NonNull<T>?

[rustbot]

r? @tgross35

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[tgross35]

This seems fine to me, but libs-api didn't specify an exact API so I'll ask them to review.

r? @Amanieu