The ONNX plugin was not receiving -DYAMS_ONNX_COREML_ENABLED=1 because
the CMake dependency method doesn't propagate COMPILE_DEFINITIONS from
Conan's pkg-config.
Changes:
- Try pkg-config first to get compile definitions with GPU flags
- Fall back to CMake method if pkg-config not available
- Add explicit platform detection fallback for macOS CoreML
- Support all GPU providers: CoreML, CUDA, DirectML, MIGraphX
Fixes GPU acceleration not being enabled despite setup.sh detecting it.
