Support taking PTX (and OpenCL-PTX) as the kernel source

[eyalroz]

Both CUDA and OpenCL allow you to use PTX as the source for a kernel, albeit somewhat differently than CUDA C++ or OpenCL C. The kernel runner should be adapted to accept PTX as input, and complete the compilation into a directly executable kernel. At least initially, we can require the user to tell us explicitly that the kernel is PTX (in addition to saying whether it's CUDA or OpenCL code), so that we don't need to worry about source file type autodetection.