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

[Nico Weber via cfe-commits]

nico wrote:

This seems to break tests: http://45.33.8.238/linux/129493/step_7.txt

Please take a look and revert for now if it takes a while to fix.


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