Get ethos-u dependency using FetchContent

[Erik-Lundell]

This change moves the dependency setup closer to where it is used, by the arm_executor_runner.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7882

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❌ 1 Cancelled Job, 2 Unrelated Failures

As of commit dc866f5 with merge base f73b8cf ():

CANCELLED JOB - The following job was cancelled. Please retry:

• trunk / test-models-macos (cmake, mv3, portable, macos-m1-stable, 90) / macos-job (gh)
  ##[error]The operation was canceled.

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• trunk / test-models-macos (cmake, emformer_transcribe, xnnpack-quantization-delegation, macos-m1-stable, 90) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[Erik-Lundell]

Unrelated CI failures.