Use std::isfinite() instead of isfinite() in portable ops.

[zingo]

Summary

This make it possible to build with a simpler libc and saves some flash space. When moving from NEWLIB_LIBC in ZephyrOS to the default one we saved almost 60 Kb in the flash memory (with a simple add model, larger models/code might save less as more libc stuff are probably used).

Test plan

Test by building ExecuTorch from ZephyrOS with and without NEWLIB_LIBC by hand.

cc @freddan80 @per @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16340

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❌ 2 New Failures, 1 Unrelated Failure

As of commit b3e30cd with merge base f3fd55c ():

NEW FAILURES - The following jobs have failed:

• Test CUDA Builds / check-all-cuda-builds (gh)
  Process completed with exit code 1.
• Test CUDA Builds / test-executorch-cuda-build-12.8 / linux-job (gh)
  RuntimeError: Command docker exec -t b274f3f9031092168416d4f020e43eab5a30adeba88c55ec2dc801798c59bedc /exec failed with exit code 1

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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