Build optimized_portable_kernels if threadpool is enabled

[swolchok]

This is step (5) of #8932.

At this exact moment, this rebuild is inefficient because it rebuilds the whole portable op library, but ops don't support optional parallelization just yet. This will become less true when we roll out parallel_for support across portable ops immediately following this PR.

[swolchok]

Stack from ghstack (oldest at bottom):

• Use parallel_for_each_reduce_over_dim_list_output_index for {Map,}ReduceOverDimListPlan ops #9197
• Skip grain_size computation in parallel_for_each_reduce_* wrappers without threadpool #9144
• Add parallel_for_each_reduce_over_dim_list_output_index #9143
• Use parallel_for_each_reduce_over_dim_output_index for {map_,}reduce_over_dim ops #9141
• Extract parallel_for_each_reduce_over_dim_output_index from argmin parallelization PoC #9139
• use numel() when appropriate in get_reduced_dim_product #9142
• Proof-of-concept: parallelize argmin #9066
• -> Build optimized_portable_kernels if threadpool is enabled #8987

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8987

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 98e3147 with merge base 4de19fd ():

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

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

• pull / unittest-arm / linux-job (gh) (trunk failure)
  backends/arm/test/models/test_conformer.py::TestConformer::test_conformer_tosa_MI

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