build_variables.bzl: split PLATFORM_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]

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❌ 1 New Failure, 1 Cancelled Job

As of commit 51478b5 with merge base f3fc096 ():

NEW FAILURE - The following job has failed:

• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 499be55fd1a234b5acc02e8114db71a1c8a40e6290c41ace9f222499948b2ae0 /exec failed with exit code 139

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.

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