build_variables.bzl: split PROGRAM_NO_PRIM_OPS_SRCS from EXECUTORCH_CORE_SRCS

[swolchok]

Making the structure in this file mirror buck's worldview more closely when it makes sense. #8268

[swolchok]

Stack from ghstack (oldest at bottom):

• build_variables.bzl: split PLATFORM_SRCS from EXECUTORCH_CORE_SRCS #8398
• -> build_variables.bzl: split PROGRAM_NO_PRIM_OPS_SRCS from EXECUTORCH_CORE_SRCS #8397
• create and validate build_variables.bzl #8326
• determine fht_avx.c/fht_neon.c based on CMake target processor #8346
• Add extension/llm/custom_ops/spinquant/test to CMake #8345
• Bypass find_package(torch) for find_package_torch_headers to help out cross compilation #8629

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8397

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 832f410 with merge base f3fc096 ():

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

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

• pull / test-llava-runner-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[swolchok]

wait, why are there 30 commits here?