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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

The "generic IR" thing is more emergent behaviour than a documented / intentional design.

This patch is fine - we shouldn't set macros to nonsense values - but if this is a step towards building libc like the rocm-device-libs there may be push back on that one.

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