This patch adds support for linking AMDGPU images using the LLD binary.
AMDGPU files are always bitcode images and will always use the LTO
backend. Additionally we now pass the default architecture found with
the amdgpu-arch tool to the argument list.
Depends on D117156
This part is valuable as-is and probably independently testable, in that I think -### will have no march= before this change and will contain march=gfx*** after it. Though for that test to work reliably, I think it would have to run on a machine where amdgpu-arch succeeded, which is probably more bother than it's worth. Will check locally.