Default CMAKE_SYSTEM_PROCESSOR to the host (#10912)

### 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](https://github.com/pytorch/executorch/blob/0a6f6220dd7252f5b79bf7b33edf1ea0631158f3/extension/llm/custom_ops/CMakeLists.txt#L52-L66)).
So, let's just explicitly set it.

### Test plan

CI

cc @larryliu0820