Need to replace semicolons with spaces when CUDA_ARCH_FLAGS is set

[eyalroz]

In the example program directory, when detecting CUDA architectures and when CUDA_ARCH_FLAGS is set, we currently adopt it as-is, while, in fact, we should replace semicolons ; with spaces, since it is a CMake list, and we need a portion of a command-line, where arguments are space-separated. I was mistakenly assuming that the replacement in the CUDA_ARCH_FLAGS-is-not-found branch would be sufficient, and it isn't.