[RFC][AMDGPU][lld] Add object linking support
Add AMDGPU ELF object-linking support in lld, including resource propagation,
LDS layout, indirect-call handling, named-barrier updates, target compatibility
checks, and kernel descriptor/metadata patching.
This is a large PR because the linker needs to understand and validate several
AMDGPU object-linking concepts end to end. I tried to keep the changes scoped to
the necessary linker support and related metadata plumbing, but I'm open to
suggestions on how to split or structure the review to make it easier.
