[clang] [AMDGPU] Do not emit arch dependent macros with unspecified cpu (PR #79660)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> LGTM. AFAIK only device libs compile OpenCL code without -mcpu. I don't think it uses any of these predefined macros.

That's what I figured from a cursory look at the ROCm-Device-Libs. The goal is to formalize this more to make more generic LLVM-IR.

https://github.com/llvm/llvm-project/pull/79660