Refactor MTK build scripts Test building MTK examples and runners in ci

[neuropilot-captain]

Summary

• Add building MTK examples to CI
• Separate MTK backend build script and example build script

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13085

• 📄 Preview Python docs built from this PR

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

❌ 3 New Failures, 66 Pending

As of commit 0997aa1 with merge base 2f782bf ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• trunk / test-arm-ootb-linux / linux-job (gh)
  RuntimeError: Command docker exec -t aefff5d7f1c184f621f351e8419342b90a20bc63f7bbf6b426d62ce11e4d9a0c /exec failed with exit code 1
• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[neuropilot-captain]

@pytorchbot label "release notes: none"

[cccclai]

Thank you for adding the CI, I believe there was a github issue for this

[neuropilot-captain]

Thank you for adding the CI, I believe there was a github issue for this

It should be fixed by the last PR: #13061
I have modified the PR summary to address the issue

Thanks!