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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Reverted. I don't think there's a "proper" solution here since this seems to have leaked into the headers due to whoever set this up initially not properly setting these on the host. That seems to be endemic now, so the best we can do it just set it to some dummy values I think.

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