Move KleidiAI support detection to CMake logic. (#26178)
### Description
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Use of the KleidiAI library is not supported on all platforms. Previously, this support detection was done in both
`tools/ci_build/build.py` and `cmake/CMakeLists.txt`. This change consolidates and simplifies the checks. They are now done in one place in the CMake logic.
Move setting of onnxruntime_target_platform into a separate file.
### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->
Clean up and simplify.
