[libc] Support setting 'native' GPU architecture for libc

We already use the `amdgpu-arch` and `nvptx-arch` tools to determine the
GPU architectures the user's system supports. We can provide
`LIBC_GPU_ARCHITECTURES=native` to allow users to easily build support
for only the one found on their system. This also cleans up the code
somewhat.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D146994