[clang] [llvm] [Offload] Change unregister library to use `atexit` instead of destructor (PR #86830)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

So, looking into `libomptarget` when this is applied is doing something weird. `atexit` is functionally a stack, so that means the first in is the last out. However, it seems that the static global constructor created inside of the CUDA plugin is being unregistered *after* this one for unknown reasons. Will need to look into that.

https://github.com/llvm/llvm-project/pull/86830