kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

This config detects if Rust and Clang have matching LLVM major version.
All IR or bitcode operations (e.g. LTO) rely on LLVM major version to be
matching, otherwise it may generate errors, or worse, miscompile
silently due to change of IR semantics.

It's usually suggested to use the exact same LLVM version, but this can
be difficult to guarantee. Rust's suggestion [1] is also major-version
only, so I think this check is sufficient for the kernel.

Link: https://doc.rust-lang.org/rustc/linker-plugin-lto.html [1]
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260203-inline-helpers-v2-1-beb8547a03c9@google.com
[ Fixed typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>