Make OS presets only build library

[jathu]

Summary

I think mixing these different motivations (i.e. pybind vs. ET lib) can causes issues in the future with conflicting flags. For example, pybind always enables EXECUTORCH_ENABLE_LOGGING regardless of build mode (#12469). So, I think we should just make the platform presets build ET targeting that platform.

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12544

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❌ 2 New Failures, 2 Pending, 1 Unrelated Failure

As of commit 83fae98 with merge base 1333b36 ():

NEW FAILURES - The following jobs have failed:

• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3ad012bf8844f3bd1e7a2c2940606022f8c4f240010d0ccaf617299751da9081 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 535b07c0a898e28eee485babf23dec03e944b46b1890bc95c9211f6d4cf5c852 /exec failed with exit code 1

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

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

• Apple / build-benchmark-app / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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