Update linker-based LTO compatibility matrix as a part of the Rustc release process

Location (URL)

https://doc.rust-lang.org/rustc/linker-plugin-lto.html#toolchain-compatibility

Summary

We already had multiple times a problem with outdated Linker-based compatibility matrix:

• https://github.com/rust-lang/rust/pull/103955
• https://github.com/rust-lang/rust/pull/146827
• https://github.com/rust-lang/rust/pull/154284

The problem is that the documentation is not updated during the Rust release process, and now it's done in some random way. That's definitely not a good way to do this.

I suggest update this page as a mandatory part of the Rustc release process - in this case, no out-of-date states will be found in the future. I don't know in which way we should do it - automatically via some script or seomhow manually - since I am not familiar with the release process. But we need to do something with it.

This information is important for Rustc users (me including), who is trying to use cross-lang LTO for Rust + C/C++ apps, we need this compatibility matrix up-to-date to not guess (aka check manually) Rustc and Clang compatibility.

Start with the linker-plugin LTO toolchain compatibility section at the linked documentation URL, then read PRs 103955, 146827, and 154284 to understand how the matrix has been updated. Next, inspect the Rustc release process to determine where this check belongs and whether it can be automated. Done means the matrix update is a defined, repeatable part of every release.