[clang] [AMDGPU] Do not emit arch dependent macros with unspecified cpu (PR #80035)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Rework of https://github.com/llvm/llvm-project/pull/79660 to handle old behavior of these being defined for the host.

https://github.com/llvm/llvm-project/pull/80035