Rust for Linux

• Update doc from Rust for Linux page to remove the superfluous --sysroot flag.
• Maybe remove well known commits from Rust for Linux page (since we now have CI).
• Specify that retpolines should be disabled in the Linux config.
• --release vs debug mismatch in the commands.
• Revert this commit: https://github.com/Rust-for-Linux/ci-rustc_codegen_gcc/commit/4cf0c8cad0fe7f63a1eb6d8679b98136b4f34d80
• Clean up this: https://github.com/Rust-for-Linux/ci-rustc_codegen_gcc/commit/c44a903c2bebe45f5917f53ca68cb3dd092512f8
• Update curl URL to download pre-compiled libgccjit.
• ARM cross-compiler for libgccjit (and test this in the CI).
• Check better comment (matching libgccjit and rustc_codegen_gcc) section when Rust 1.76 is released.
• Fix panic when dumping IR GCC.
• Longarch cross-compiler and tests?

Start with the Rust for Linux page and the CI workflow at .github/workflows/ci.yaml, then review the two referenced commits and the remaining checklist items. Done means the documentation and configuration updates are resolved, the ARM and Longarch cross-compiler paths are addressed or clarified, and the relevant CI tests pass.