[LLVM] Simplify GPU runtimes flag handling (#159802)

Summary:
The AMDGPU hack can be removed, and we no longer need to skip 90% of the
`HandleLLVMOptions` if we work around NVPTX earlier. Simplifies the
interface by removing duplicated logic and keeps the GPU targets from
being weirdly divergent on some flags.
GitOrigin-RevId: 2d503b924c324f7f67ef8b1f5c518c1f80a2825b