Default CMAKE_SYSTEM_PROCESSOR to the host

[jathu]

Summary

Although by default cmake use the host's architecture if CMAKE_SYSTEM_PROCESSOR isn't provided, it doesn't actually set the value. This can be an issue in cases where people try to read this value (i.e. extension/llm/custom_ops/CMakeLists.txt). So, let's just explicitly set it.

Test plan

CI

cc @larryliu0820

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 31a61d2 with merge base 47164cc ():

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)
  test_llava_export

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