Move KleidiAI support detection to CMake logic.

[edgchen1]

Description

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.

Motivation and Context

Clean up and simplify.

[snnn]

Hello! I've reviewed the changes, and this is a great refactoring that simplifies the build logic. I have a few technical suggestions for improvement.

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1. Potential bug in KleidiAI platform detection for Apple

In cmake/CMakeLists.txt, the platform check for Apple is:

if(NOT (... OR (APPLE AND CMAKE_OSX_ARCHITECTURES STREQUAL "arm64")))

This check depends on CMAKE_OSX_ARCHITECTURES being defined. According to the logic in the new cmake/detect_onnxruntime_target_platform.cmake file, if CMAKE_OSX_ARCHITECTURES is not defined during a build on an Apple machine, onnxruntime_target_platform is set from CMAKE_SYSTEM_PROCESSOR. For a native build on Apple Silicon, this means onnxruntime_target_platform would be arm64, but CMAKE_OSX_ARCHITECTURES would be undefined.

In this case, the check above would evaluate to false, and KleidiAI would be incorrectly disabled.

A more robust condition would be to check onnxruntime_target_platform directly:

if(NOT (... OR (APPLE AND onnxruntime_target_platform STREQUAL "arm64")))

This would correctly enable KleidiAI for native Apple Silicon builds, regardless of whether CMAKE_OSX_ARCHITECTURES is explicitly set.

2. Potential issues in cmake/adjust_global_compile_flags.cmake

I noticed two small things in this file:

• The if/elseif chain appears to have a syntax error based on the indentation in the diff. It looks like an if statement was added after an elseif, but before the final else, which is not valid CMake syntax. The logic for enabling SAFESEH for x86 builds should likely be nested inside the elseif block that checks for x64 or x86, or placed after the entire if-elseif-else block.

• The FATAL_ERROR message at the end of that block seems to have a copy-paste error:

  message(FATAL_ERROR "Unknown CMAKE_SYSTEM_PROCESSOR: ${CMAKE_SYSTEM_PROCESSOR}")

  Since the variable being checked throughout this block is onnxruntime_target_platform, the error message should probably refer to that variable for clearer debugging:

  message(FATAL_ERROR "Unknown onnxruntime_target_platform: ${onnxruntime_target_platform}")

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