add llvm writable attribute conditionally This PR tries to address https://github.com/rust-lang/unsafe-code-guidelines/issues/584#issue-3440604811. It is part of a bachelor thesis supervised by @JoJoDeveloping and @RalfJung, for more information, see: [Project_Description.pdf](https://github.com/user-attachments/files/26537277/Project_Description.pdf). If the new `-Zllvm-writable` flag is set, the [llvm writable attribute](https://llvm.org/docs/LangRef.html#writable) is inserted for all mutable borrows. This can be conditionally turned off on a per-function basis using the `#[rustc_no_writable]` attribute. The new Undefined Behaviour introduced by this can detected by Miri, which is implemented here: https://github.com/rust-lang/miri/pull/4947. Two library functions already received the `#[rustc_no_writable]` attribute, as they are known to cause problems under the Tree Borrows aliasing model with implicit writes enabled.