Add compiler/cross package for cross-compilation libc++ libraries

[Copilot]

• compiler/cross/BUILD — cmake targets from @llvm_source with cross-compile target triple flags, like msan/tsan. No prebuilt downloads.
• MODULE.bazel — remove @llvm_prebuilt_aarch64 and @llvm_prebuilt_x86_64 http_archives
• versions.bzl — remove "llvm_prebuilt_sha256" dict (not needed)
• compiler/cross/libcxx_cross.bzl — fix generated BUILD to use flat lib/ paths (cmake output layout)
• compiler/cross/README.md — update to reflect cmake approach
• bazel.yml — both arch artifacts uploaded (already correct)

Original prompt

Context

PR #3779 proposes overlaying aarch64 libc++ into the sysroot when cross-compiling for arm64. However, the sysroot should stay lean — it's there to provide glibc headers/libs and nothing more. We need a better approach.

What to implement

Create a standalone extraction package for cross-compilation libc++ libraries, following the exact same pattern as the existing msan_libs/tsan_libs in bazel/compile/sanitizer_libs.bzl. The key insight is that libc++.a, libc++abi.a, and libunwind.a are already prebuilt in the official LLVM release tarballs — no compilation needed.

Files to create/modify

1. Create bazel/compile/libcxx_cross.bzl

Create a new repository rule libcxx_cross that:

• Takes target_arch (e.g., "aarch64", "x86_64"), llvm_version, and sha256 as attributes
• Maps target arch to the LLVM release triple (e.g., "aarch64" → "aarch64-linux-gnu", "x86_64" → "x86_64-linux-gnu-ubuntu-18.04")
• Downloads the official LLVM release tarball from https://github.com/llvm/llvm-project/releases/download/llvmorg-{ver}/clang+llvm-{ver}-{triple}.tar.xz
• Extracts only libc++.a, libc++abi.a, libunwind.a from the lib/{target-unknown-linux-gnu}/ subdirectory of the tarball
• Also extracts the __config_site header from include/{target-unknown-linux-gnu}/c++/v1/__config_site
• Generates a BUILD.bazel that exposes:
  • cc_library targets for individual libs (:libcxx, :libcxxabi, :libunwind)
  • A combined cc_library target (:libcxx_cross) that bundles all three
  • A filegroup for the __config_site header
  • All with visibility = ["//visibility:public"]
• Also create a setup_libcxx_cross() helper function (similar to setup_sanitizer_libs()) that uses defaults from VERSIONS

Follow the style/patterns of the existing bazel/compile/sanitizer_libs.bzl closely. Reference the existing code at https://github.com/envoyproxy/toolshed/blob/05dcb996bb8a087a88eb7627e05f99f327f8830b/bazel/compile/sanitizer_libs.bzl for the pattern.

The tarball structure for LLVM 18.1.8 aarch64 is:

clang+llvm-18.1.8-aarch64-linux-gnu/
  lib/aarch64-unknown-linux-gnu/libc++.a
  lib/aarch64-unknown-linux-gnu/libc++abi.a  
  lib/aarch64-unknown-linux-gnu/libunwind.a
  include/aarch64-unknown-linux-gnu/c++/v1/__config_site

2. Update bazel/compile/extensions.bzl

Add a new module extension libcxx_cross_extension (following the same pattern as sanitizer_extension) that allows bzlmod consumers to configure libcxx_cross in their MODULE.bazel.

3. Update bazel/versions.bzl

Add SHA256 hashes for the LLVM release tarballs used by this package. Add a new dict keyed by arch, e.g.:

"libcxx_cross_sha256": {
    "aarch64": "dcaa1bebbfbb86953fdfbdc7f938800229f75ad26c5c9375ef242edad737d999",
},

This is the sha256 of the clang+llvm-18.1.8-aarch64-linux-gnu.tar.xz release tarball.

4. Update bazel/compile/BUILD

Add the new .bzl file to any relevant exports/filegroups if needed to match the existing pattern.

5. Update bazel/MODULE.bazel

Add a commented-out example (like the existing sanitizer_ext example) showing how downstream consumers would use the libcxx_cross extension:

# Setup cross-compilation libc++ libraries
# libcxx_cross_ext = use_extension("@envoy_toolshed//compile:extensions.bzl", "libcxx_cross_extension")
# libcxx_cross_ext.setup()
# use_repo(libcxx_cross_ext, "libcxx_cross")

6. Update bazel/compile/README.md

Add documentation for the new libcxx_cross package, covering:

• What it provides and why (cross-compilation libc++ without polluting the sysroot)
• How to use with WORKSPACE
• How to use with bzlmod

Design principles

• Sysroot stays clean: This is a separate repository, not part of the sysroot
• Arch-generic: Parameterized by target_arch, not hardcoded to arm64
• Opt-in: Only fetched/instantiated when a consumer needs it
• No compilation needed: Just download-and-extract from LLVM release tarballs
• Follows existing patterns: Matches msan_libs/tsan_libs structure exactly

This pull request was created from Copilot chat.

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