[libc] Add code for detecting NVIDIA GPUs as well

Recently the `nvptx-arch` tool was added to address the lack of a
similar tool for detecting locally installed NVIDIA GPUs. This patch
adds similar functionality for the already existing `amdgpu-arch` tool
in libc. These will be used to run tests on the user's system in the
future. On a system with both GPUs installed we will just go with the
first GPU detected. In the future we may want to configure the tests to
run on both of them.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D142776