[llvm] [AMDGPU] Add AMDGPU support for llvm-objcopy (PR #92066)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

I agree that this should probably go in `cross-arch-headers.test` alongside the other targets, unless there is a good reason not to.

That being said, I'm not sure I agree with having the GPU name in the -O flag unless it's done dynamically. I'd like to avoid enumerating all GPUs in yet another place if it can be avoided. So either populate the map automatically, or parse the target into a flag, etc.

https://github.com/llvm/llvm-project/pull/92066