Add OSS CI to cross-compile and run the Cadence Xtensa backend

[aliafzal]

Summary

Adds OSS CI to cross-compile and run the Cadence (Xtensa) backend, so backend changes can be validated in CI rather than only through internal flows.

• gflags-free ISS runner (cadence_executor_runner): the upstream executor_runner can't cross-compile to Xtensa (gflags pulls in mkdir(2), absent from Xtensa newlib). This uses argv-only parsing like the Arm/NXP backends, loads a .pte, and runs the first method on the simulator. Also registers the quantized_depthwise_conv1d ops the HiFi4 codegen references but the operators CMake omitted (broke the link).
• xtensa-build job added to the existing Cadence Build & Test workflow (build-cadence-runner.yml), alongside the host cpu-build/cpu-test. It cross-compiles the runner for the hifi4 and vision cores and uploads it; an ISS test stage follows later (cf. cpu-build -> cpu-test). The licensed toolchain/core artifacts are fetched at runtime from an auth-gated store via a short-lived OIDC credential; role/region/store are supplied via CI variables and are not committed.

fusion_g3 is omitted from the matrix until an upstream operator/nnlib API skew is resolved (its runner does not link yet).

Test plan

Validated locally end to end: setup + cross-compile produce an Xtensa ELF runner for vision; the HiFi4 op sweep passes 15/15 on the simulator and a trivial model returns the expected output.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20208

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❌ 2 New Failures, 1 Pending, 1 Unrelated Failure, 2 Unclassified Failures

As of commit 4916383 with merge base 5526971 ():

NEW FAILURES - The following jobs have failed:

• pull / test-sqnr-static-llm-qnn-linux (smollm2_135m) / linux-job (gh)
  RuntimeError: Command docker exec -t 3879b28d6c0fa49e4de9183322155fbd9fb00009ab81b1945a79a701666288f5 /exec failed with exit code 92
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 9f24aa82dd3e7eab0f8520e351929daefeab1482941b34b93186100da305d736 /exec failed with exit code 1

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Process completed with exit code 1.
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64

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

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

• pull / android / build-android (gh) (trunk failure)
  Process completed with exit code 1.

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