Draft: Add extended support from new Neutron C flow for Clamp operator

[StrycekSimon]

Summary

• Moves flag indicating use of the new Neutron C flow from CustomCompileConfig to NeutronTargetSpec
• Adds new Neutron C flow support for Clamp operator

Test plan

New test cases for Clamp are introduced. The relocation of new flag is covered by already existing unit tests.

cc @robert-kalmar @JakeStevens @digantdesai @rascani

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19510

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❌ 2 New Failures, 1 Pending, 3 Unrelated Failures

As of commit 3a90b03 with merge base b04cc65 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 79da876d9396bfef64b2ca59c2ca2395728e11e296e8fa4715d8d008df4d6b0b /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  ../../dev/ir/converter/node_converter/test_mul_tensor_converter.py::TestMulTensorNewNeutronFlow::test__w_conv_unsupported[2

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)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe

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