[rust-lang/rust] Cranelift patch files are painful to maintain

### ROOT CAUSE The use of patch files in `rustc_codegen_cranelift/patches/` creates a brittle and error-prone maintenance process. Patch files are fragile because they require manual intervention to apply and maintain alignment with upstream changes. This is exacerbated by CI checks that fail if patches don't apply, leading to contributor frustration. The underlying issue is the lack of a more robust mechanism for code differentiation between the Rust compiler and Cranelift-specific adaptations. ### CODE FIX Replace patch files with conditional compilation in the standard library and compiler crates. Define a `cfg(cranelift)` flag to conditionally exclude or adjust code incompatible with Cranelift. For example: ```rust // In std crate (or relevant compiler crates) #[cfg(not(cranelift))] fn unsupported_feature() { // Code incompatible with Cranelift } ``` 1. **Add Configuration Flag**: Modify `compiler/rustc_codegen_cranelift/build.rs` to propagate `--cfg cranelift` to dependent crates. 2. **Refactor Code**: Replace patches with `cfg(cranelift)` annotations in `compiler/rustc_codegen_cranelift` and standard library code. 3. **Update CI**: Adjust CI to pass `--cfg cranelift` when building with Cranelift. This approach aligns with Miri's `--cfg miri` model, reducing patch file dependency and improving maintainability.