[LLVM] Port 'llvm-gpu-loader' to use LLVMOffload (#162739)
Summary:
This patch rewrites the `llvm-gpu-loader` utility to use the LLVMOffload
interface. This heavily simplifies it while re-using the already
existing support. Another benefit is that I can now easily do this
dynamically so we can always build this utility without needing to find
non-standard packages.
One issue is mentioned in
https://github.com/llvm/llvm-project/issues/159636 where this will now
take extra time if you have both installed on the same machine. This is
just slightly annoying since most people don't have both CUDA and ROCm
at the same time so I don't consider it a blocker. I will work later to
address it.
Slightly unfortunate environment variable usage, I will also expose that
better in the future.
Fixes: https://github.com/llvm/llvm-project/issues/132890
