Add Buck support for right-sizing operator registry

[rascani]

Summary:
Adds Buck support for auto-right-sizing the ExecuTorch operator registry, matching the CMake feature landed in #19118.

A new operator_registry_selective() macro mirrors prim_ops_registry_selective(): a genrule stages operator_registry.cpp, operator_registry.h, and a generated selected_max_kernel_num.h in one artifact tree, and a per-binary cxx_library compiles them together. operator_registry.cpp's existing __has_include ladder picks up EXECUTORCH_SELECTED_MAX_KERNEL_NUM from the generated header.

executorch_generated_lib gains an opt-in auto_size_kernel_registry parameter (default False). The operator registry is intentionally global, so the per-binary variant defines the same external-linkage symbols as the shared //executorch/runtime/kernel:operator_registry. Turning the flag on requires the consumer to ensure no transitive dep on the shared target. User
-c executorch.max_kernel_num=N still wins via the existing preprocessor flags.

When include_all_prim_ops=False, the binary already builds its own selective prim ops variant; the counter receives the aggregated selected_prim_ops.h(via the new --selected-prim-ops-header flag on gen_max_kernel_num.py) so prim ops compiled out under ET_PRIM_OPS_SELECTIVE_BUILD aren't counted.

Fixes #19512

Differential Revision: D104440331

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❌ 10 New Failures, 6 Unrelated Failures

As of commit 5505272 with merge base 8020fe0 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
• periodic / test-models-linux (buck2, mv3, portable, linux.2xlarge, 90) / linux-job (gh)
  RuntimeError: Command docker exec -t 6cb8c566b735d0847f6fdc24d74c4e737a774a46361c75d61a0060f857cc5a95 /exec failed with exit code 3
• periodic / test-models-linux (buck2, mv3, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job (gh)
  RuntimeError: Command docker exec -t 5aa26148e95f22a761942ce3d8d4637d12248e1db3b55934ef1532788b96a557 /exec failed with exit code 3
• pull / test-models-linux-basic (mv3, portable, buck2, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh)
  RuntimeError: Command docker exec -t 046208607be936672c3fc3d69696c90f7b2f9494201db0ac1d02451c5c724270 /exec failed with exit code 3
• pull / test-models-linux-basic (mv3, xnnpack-quantization-delegation, buck2, linux.2xlarge, executorch-u... / linux-job (gh)
  RuntimeError: Command docker exec -t fc962e6fa975c0d751007d0cc443ec61bca167c6b3cd2bb5ae1e0c7582f3ae94 /exec failed with exit code 3
• pull / test-models-linux-basic (vit, portable, buck2, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh)
  RuntimeError: Command docker exec -t 7b6f2db8d77168aefb93e2dbbdac4907009e3551c335efaaaef4f651eb957f22 /exec failed with exit code 3
• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, buck2, linux.2xlarge, executorch-u... / linux-job (gh)
  RuntimeError: Command docker exec -t b701b5a8a3ad93fbe5abce7db901a29eea3ed88716439bad6ed483ef74e7d6b0 /exec failed with exit code 3
• pull / test-qnn-buck-build-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6241ed1946473e71cfa5e0b311a8a73b0b66c7584509e2f75504c4d32f291f42 /exec failed with exit code 3
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3eeecc95693a02f0913bf6ff0d4e69e1ddc87fb4ff1e5bb15a3179a6c99cd9ed /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / unittest / linux / linux-job (gh) (similar failure)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1
• pull / unittest-editable / linux / linux-job (gh) (similar failure)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1
• pull / unittest-editable / macos / macos-job (gh) (similar failure)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1

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)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1
• pull / unittest / windows / windows-job (gh) (trunk failure)
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)

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