[codex] arm64e upstream prep stack

[cypherair]

Summary

Rebuilds the arm64e ptrauth spike into an eight-commit upstream-prep stack on top of main at 36ba2c7712052d731a7082d0eba5ed3d9d56c133.

This is intentionally a CypherAir fork draft PR. The upstream-core cut point is before the final fork-only workflow commit; do not send this exact branch to rust-lang/rust unchanged.

Stack

1. target: add arm64e-apple-visionos
2. arm64e: model default Apple ptrauth features
3. arm64e: emit clang-compatible ptrauth metadata
4. arm64e: authenticate indirect function calls
5. arm64e: authenticate function pointers used as data
6. arm64e: reject incompatible ptrauth flag overrides
7. bootstrap: handle fork shallow upstream detection
8. ci: add fork arm64e validation workflow

The final workflow commit is fork-only and marked to drop before upstream. The bootstrap shallow-ref commit is separate from arm64e codegen so it can be proposed independently or dropped.

Validation

• git diff --check main...HEAD
• python3 x.py check compiler/rustc_codegen_ssa compiler/rustc_codegen_llvm --stage 1
• Full targeted arm64e codegen/assembly/UI test command, including fnptr-data and invoke
• python3 x.py build compiler/rustc library/std --stage 1 --target arm64e-apple-darwin
• python3 x.py test src/bootstrap --stage 1 --test-args test_push_ci
• python3 x.py test src/bootstrap --stage 1 --test-args test_ci_merge_without_upstream_parent_falls_back_to_nightly_ref
• Scratch stage1 arm64e-apple-darwin samples: hello world, thread_local!, and std::thread::current().id() all compiled to Mach-O arm64e and ran locally; TLS assembly contains blraaz.

python3 x.py test src/bootstrap --stage 0 is rejected by this checkout (cannot test anything on stage 0). A full src/bootstrap --stage 1 run is blocked locally by missing cmake, ninja, and Linux cross compilers, so the new git-helper tests were run directly.

Notes

• TLS is not a current blocker for the minimal stage1 samples.
• The gcc backend skips are retained on the LLVM/AArch64-specific UI tests with local rationale comments.
• Local ignored status notes were updated in the Rust checkout and downstream experiment notes, but are not part of this PR stack.

[cypherair]

Superseded by PR #5, which now carries the upstream-ready integration stack plus the integration-only visionOS+ptrauth cross-check follow-up. Keeping the branch for archive/reference history.