Pass deployment target when linking metallib

[dhiltgen]

MLX compiles each Metal kernel AIR file with -mmacosx-version-min from CMAKE_OSX_DEPLOYMENT_TARGET, but the final metal driver invocation did not receive that deployment target.

With the macOS 26 SDK and CMAKE_OSX_DEPLOYMENT_TARGET=14.0, the metal driver otherwise invokes air-lld with the SDK default platform version and can stamp mlx.metallib as macOS 26 even though the input AIR files target macOS 14.

Repro:

cmake -S . -B build-macos14 -DCMAKE_BUILD_TYPE=Release -DCMAKE_OSX_DEPLOYMENT_TARGET=14.0

cmake --build build-macos14 --target mlx-metallib --parallel

strings -a build-macos14/mlx/backend/metal/kernels/mlx.metallib | grep -Eo 'air64[^[:space:]]*macosx[0-9.]+' | sort -u

Keep using metal as the linker front-end and pass -mmacosx-version-min through the final link step. Verified with macOS SDK 26.4 that the resulting library reports air64_v26-apple-macosx14.0.0.

Proposed changes

Please include a description of the problem or feature this PR is addressing. If there is a corresponding issue, include the issue #.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[zcbenz]

Looks good to me, thanks!