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

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

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

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