Use optimized kernels when available in top-level selective build

[GregoryComer]

Summary

Use optimized kernels for the top-level executorch_selected_kernels target when built.

Test plan

I've manually built with top-level selective build to verify that this works. CI coverage will be provided with #13741, which should land shortly.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14203

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 4 Cancelled Jobs, 3 Unrelated Failures

As of commit f42c7e3 with merge base 6ed10e5 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• periodic (gh)
• pull / test-binary-size-linux-gcc / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh) (trunk failure)
  RuntimeError: Failed to install QNN SDK. Please check the logs above.
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh) (trunk failure)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64
• pull / test-moshi-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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