Tracking issue for MCP 994: Add `-Zllvm-target-feature` target *modifier* to directly set LLVM-level target features, and deprecate doing that with `-Ctarget-feature`

This is a tracking issue for MCP "Add -Zllvm-target-feature target modifier to directly set LLVM-level target features, and deprecate doing that with -Ctarget-feature" (rust-lang/compiler-team#994).

[!NOTE]
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Steps

• Implement the MCP
  • Initial implementation: https://github.com/rust-lang/rust/pull/160457
  • Implement the special target modifier checks: https://github.com/rust-lang/rust/pull/160618
• Adjust the documentation
  • In the Rust Compiler Development Guide if applicable (e.g., for refactorings)
  • In the Rust Forge if applicable (e.g., for policy changes)
  • In the rustc book if applicable (e.g., for target tier or CLI changes)
  • In the Unstable Book if applicable (e.g., for unstable CLI changes)

Unresolved questions

None so far.

Implementation history

Empty so far.

@RalfJung, consider @rustbot claim'ing the work.

Start with the tracking checklist and review the initial implementation in PR 160457, then inspect the remaining special target modifier checks in PR 160618. Check whether the Rust Compiler Development Guide, Rust Forge, rustc book, or Unstable Book needs documentation updates; done means the implementation checks and applicable documentation are complete.