[NATIVE_CPU][SYCL] Switch to using native_cpu compiler pipeline inline instead of OCK fetchContent

[coldav]

Bring Native CPU pipeline files into dpc++ directly.

This brings the Native CPU compiler pipeline files directly under the ownership of intel/llvm. This removes the direct dependence on the oneAPI Construction Kit, although the history of those files still exists under intel/llvm and the originals still exist at https://github.com/uxlfoundation/oneapi-construction-kit.

This was done as follows:

In the oneAPI Construction Kit repo:

    git-filter-repo --paths-from-file $script_dir/native_cpu_paths.txt \
    --path-rename modules/compiler/compiler_pipeline:llvm/lib/SYCLNativeCPUUtils/compiler_passes/compiler_pipeline \
    --path-rename modules/compiler/vecz:llvm/lib/SYCLNativeCPUUtils/compiler_passes/vecz \
    --path-rename modules/compiler/multi_llvm/include/multi_llvm:llvm/lib/SYCLNativeCPUUtils/compiler_passes/compiler_pipeline/include/multi_llvm

See uxlfoundation/oneapi-construction-kit#969 for more info.

Under intel/llvm:

    git checkout sycl
    git checkout -b $llvm_branch
    git fetch $ock_fork $ock_branch
    # Update CMake to be more llvm like
    # clang-format-20 on added files
    # Add some new documentation to reflect the state of play.
    git merge -m "[NATIVE_CPU][SYCL] Merge from oneAPI Construction Kit into native_cpu" --allow-unrelated-histories ock_fork/$ock_branch

llvm/lib/SYCLNativeCPUUtils/CMakeLists.txt was updated to remove the fetch content and the ugly install workaround. Missing is the lit tests for the pipeline tests as these refer to the use of a non-ported tool, muxc. CMakeLists.txt across was updated to use LLVM macros across compiler_pipeline and vecz ported code, as well as changing NATIVECPU_USE_OCK to be the default. It also applies clang-format-20 to all the .h and .cpp files brought in. Other than that the .cpp and .h files are unchanged. The lit tests cfg files were updated to fit with LLVM, and a small change was made to avoid use of undef.

A brief explanation and limitations of the importing of the files are under llvm/lib/SYCLNativeCPUUtils/compiler_passes/compiler_passes.rst.

This was based on the oneAPI Construction Kit as hash d3b68da068110b6f3858769ae7ce8c5f6750705b

[coldav]

Note: I've removed most of the docs in favour of a separate PR under sycl/docs.

[aelovikov-intel]

@intel/dpcpp-nativecpu-reviewers , this seems to be blocked by your review.

This needs to be merged and not squashed. Removing my approval to prevent accidental squash.

[aelovikov-intel]

/merge

[sarnex]

/merge