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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> Hi, Would it be okay to add -DELF_FLAGS to the yaml2obj invocations, it will be set to "" for every command invocation, except for AMDGPU, where we set it to EF_AMDGPU_MACH_AMDGCN_GFX900?

I think that works, @jh7370 ?

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