[CI/Build] fix cpu_extension for apple silicon

[ignaciosica]

Purpose

The purpose for this pr is to fix the cpu installation from source for apple silicon CPUs. Recently, basic serving capabilities and examples stopped working in my local setup (M3 pro). I bisected my way to this pr #14129 which introduced int8 quantization for ARM CPU. The problem lied on the fact that although apple silicon shared some codepath with the rest of the arm cpu in cpu_extension.cmake, it also had some specific configurations that ended up breaking build after the int8 support was introduced; more specifically, apple silicon pathway was not enabling ASIMD_FOUND thus not including quant.cpp (cpu_extension.cmake:284) source.

After a fresh install from source

git clone https://github.com/vllm-project/vllm.git
cd vllm
uv venv --python 3.12 --seed
source .venv/bin/activate
uv pip install -r requirements/cpu.txt
uv pip install -e .

Basic example failed with

python examples/offline_inference/basic/basic.py
> WARNING 07-18 12:13:35 [_custom_ops.py:20] Failed to import from vllm._C with ImportError("dlopen([...]/vllm/vllm/_C.abi3.so, 0x0002): symbol not found in flat namespace '__Z14int8_scaled_mmRN2at6TensorERKS0_S3_S3_S3_RKNSt3__18optionalIS0_EE'")
> [...]
> AttributeError: '_OpNamespace' '_C_cache_ops' object has no attribute 'reshape_and_cache'

In order to fix, this pr enabled ASIMD_FOUND for apple silicon as well. For safety, it checked for support with the following command: sysctl -n hw.optional.neon. As far as I know, all apple silicon, starting from M1 up to M4 generation support this feature, but still decided to gate ASIMD_FOUND on this check in the case the support is dropped for future generations.

After this fix, cpu installation from source started working again.

This pr also enables bf16 support for apple silicon. This feature is gated via the following check hw.optional.arm.FEAT_BF16. Based on this LLVM's commit, bf16 support was introduced for cpu in m2 generation.

Test Plan

Unfortunately I'm not familiar enough with build runs in CI, I would appreciate some guidance for this point.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request fixes a build issue for Apple Silicon CPUs by enabling ASIMD support and introducing a check_sysctl function. To improve robustness, I've identified a critical issue regarding argument quoting in the new check_sysctl function that should be addressed.

[ignaciosica]

addressed commit signing issue

[ignaciosica]

removed APPLE_SILICON_FOUND as it wasn't used downstream anymore but maybe it's still useful to keep it for future reference/use?

[mgoin]

Yeah this would be useful to follow the pattern and use in downstream code

[mgoin]

LGTM I think this is reasonable and shouldn't be disruptive to other backends