Rust changes for v6.11

The highlight is the establishment of a minimum version for the Rust
toolchain, including 'rustc' (and bundled tools) and 'bindgen'.

The initial minimum will be the pinned version we currently have, i.e.
we are just widening the allowed versions. That covers 3 stable Rust
releases: 1.78.0, 1.79.0, 1.80.0 (getting released tomorrow), plus beta,
plus nightly.

This should already be enough for kernel developers in distributions
that provide recent Rust compiler versions routinely, such as Arch
Linux, Debian Unstable (outside the freeze period), Fedora Linux,
Gentoo Linux (especially the testing channel), Nix (unstable) and
openSUSE Slowroll and Tumbleweed.

In addition, the kernel is now being built-tested by Rust's pre-merge
CI. That is, every change that is attempting to land into the Rust
compiler is tested against the kernel, and it is merged only if it
passes. Similarly, the bindgen tool has agreed to build the kernel in
their CI too.

Thus, with the pre-merge CI in place, both projects hope to avoid
unintentional changes to Rust that break the kernel. This means that,
in general, apart from intentional changes on their side (that we
will need to workaround conditionally on our side), the upcoming Rust
compiler versions should generally work.

In addition, the Rust project has proposed getting the kernel into
stable Rust (at least solving the main blockers) as one of its three
flagship goals for 2024H2 [1].

I would like to thank Niko, Sid, Emilio et al. for their help promoting
the collaboration between Rust and the kernel.

[1] https://rust-lang.github.io/rust-project-goals/2024h2/index.html#flagship-goals

Toolchain and infrastructure:

 - Support several Rust toolchain versions.

 - Support several bindgen versions.

 - Remove 'cargo' requirement and simplify 'rusttest', thanks to 'alloc'
   having been dropped last cycle.

 - Provide proper error reporting for the 'rust-analyzer' target.

'kernel' crate:

 - Add 'uaccess' module with a safe userspace pointers abstraction.

 - Add 'page' module with a 'struct page' abstraction.

 - Support more complex generics in workqueue's 'impl_has_work!' macro.

'macros' crate:

 - Add 'firmware' field support to the 'module!' macro.

 - Improve 'module!' macro documentation.

Documentation:

 - Provide instructions on what packages should be installed to build
   the kernel in some popular Linux distributions.

 - Introduce the new kernel.org LLVM+Rust toolchains.

 - Explain '#[no_std]'.

And a few other small bits.
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