[libc] Enable integration tests targeting NVIDIA GPUs

This patch adds the necessary build infrastructure to build and run the
integration tests on NVIDIA GPUs. The NVIDIA `nvlink` linker utility is
what is ultimately used to combine these files into a single executable
image. Unfortunately, their tool does not support static libraries. So
we need to link with every object directly instead. This could be solved
by impelementing a "wrapper" utility around `nvlink` like we used to use
for OpenMP. But for now this should be sufficient.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D146861