set CMAKE_SYSTEM_PROCESSOR in xnnpack.cmake (#22155)
### Description
<!-- Describe your changes. -->
### Motivation and Context
By default, CMAKE_SYSTEM_PROCESSOR is same CMAKE_HOST_SYSTEM_PROCESSOR
https://cmake.org/cmake/help/latest/variable/CMAKE_SYSTEM_PROCESSOR.html
KleidiAI uses CMAKE_SYSTEM_PROCESSOR to determine whether to include
some arm64 ukernels.
https://gitlab.arm.com/kleidi/kleidiai/-/blob/main/CMakeLists.txt#L134
We use Mac with Intel CPU to cross compile MAC with ARM in ios packaging
pipeline
So we need to make CMAKE_SYSTEM_PROCESSOR same with ORT_TARGET_PROCESSOR
