create and validate build_variables.bzl

[swolchok]

First step of #8268 -- we are moving from buck-extracted filelist to using one shared filelist, and the first step is to create the shared filelist and validate it against the buck generation.

[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/8326

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ You can merge normally! (1 Unrelated Failure)

As of commit 7f5cfd3 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.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your changes are user facing and intended to be a part of release notes, please use a label starting with release notes:.

If not, please add the topic: not user facing label.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "topic: not user facing"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.

[swolchok]

Current error:

Buck-generated _custom_ops__srcs does not match hardcoded CUSTOM_OPS_SRCS
    in build_variables.bzl.  Left:
    extension/llm/custom_ops/op_fallback.cpp;extension/llm/custom_ops/op_fast_hadamard_transform.cpp;extension/llm/custom_ops/op_sdpa.cpp;extension/llm/custom_ops/op_update_cache.cpp;extension/llm/custom_ops/spinquant/fast_hadamard_transform.cpp;extension/llm/custom_ops/spinquant/third-party/FFHT/fht_avx.c;kernels/portable/cpu/util/reduce_util.cpp


     Right: extension/llm/custom_ops/op_fallback.cpp;extension/llm/custom_ops/op_fast_hadamard_transform.cpp;extension/llm/custom_ops/op_sdpa.cpp;extension/llm/custom_ops/op_update_cache.cpp;extension/llm/custom_ops/spinquant/fast_hadamard_transform.cpp;extension/llm/custom_ops/spinquant/third-party/FFHT/fht_neon.c;kernels/portable/cpu/util/reduce_util.cpp

So, we need a mechanism for choosing fht_avx.c on Intel and fht_neon.c on ARM. Best way is probably to add a CMake-native way to do this and filter these files out of the Buck-to-CMake pipeline.