[libc] Search for the CUDA patch explicitly when testing

The packaged version of the `libc` library does not depend on the CUDA
installation because it only uses `clang` and emits LLVM-IR. However,
for testing we directly need the CUDA toolkit to emit and execute the
files. This patch explicitly passes `--cuda-path` to the relevant
compilations for NVPTX testing.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D147653