[libc] Forward CUDA options to the runtimes invocation of `libc`

Some configurations may require `-DCUDAToolkit_ROOT` to find CUDA
properly. This is currently not forwarded to the CMake invocation. This
patch adds a prefix so it will be visible when the runtimes build is
started.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D147582