[libc] Add the '--threads' and '--blocks' option to the GPU loaders

We will want to test the GPU `libc` with multiple threads in the future.
This patch adds the `--threads` and `--blocks` option to set the `x`
dimension of the kernel. Using CUDA terminology instead of OpenCL for
familiarity.

Depends on D148288 D148342

Reviewed By: jdoerfert, sivachandra, tra

Differential Revision: https://reviews.llvm.org/D148485