Fix CMake cross-compiling
Fix CMake cross-compiling

Without specifying thread count, NUM_THREADS would not be defined and CMake would fail.
This is because core count cannot be determined when cross-compiling.
xianyi/OpenBLAS,kortschak/OpenBLAS,xianyi/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,kortschak/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS