support lower compute capability
Trying to build for my gt540m which is compute 2.1 but gives below error

```
[ 24%] Built target jansson
[ 26%] Building NVCC (Device) object CMakeFiles/xmrig-cuda.dir/src/nvidia/xmrig-cuda_generated_cuda_extra.cu.o
nvcc fatal   : Unsupported gpu architecture 'compute_60'
CMake Error at xmrig-cuda_generated_cuda_extra.cu.o.cmake:207 (message):
  Error generating
  /home/shensi/xmrig-nvidia/build/CMakeFiles/xmrig-cuda.dir/src/nvidia/./xmrig-cuda_generated_cuda_extra.cu.o


CMakeFiles/xmrig-cuda.dir/build.make:70: recipe for target 'CMakeFiles/xmrig-cuda.dir/src/nvidia/xmrig-cuda_generated_cuda_extra.cu.o' failed
make[2]: *** [CMakeFiles/xmrig-cuda.dir/src/nvidia/xmrig-cuda_generated_cuda_extra.cu.o] Error 1
CMakeFiles/Makefile2:105: recipe for target 'CMakeFiles/xmrig-cuda.dir/all' failed
make[1]: *** [CMakeFiles/xmrig-cuda.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

```