cmake: don't define ENABLE_NEON flags in non-cross-compiling mode
cmake: don't define ENABLE_NEON flags in non-cross-compiling mode

- NEON / ANDROID_ARM_NEON are toolchain-specific flags
- they are usually not defined for native builds
- let work CPU_BASELINE=DETECT properly

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv