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

[Joseph Huber via cfe-commits]

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@@ -175,6 +175,8 @@ Predefined Macros
      - Defined when the GPU default stream is set to per-thread mode.
    * - ``HIP_API_PER_THREAD_DEFAULT_STREAM``
      - Alias to ``__HIP_API_PER_THREAD_DEFAULT_STREAM__``. Deprecated.
+   * - ``__AMDGCN_WAVEFRONT_SIZE__``
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jhuber6 wrote:

No, it's explicitly wrong, but there's a lot of HIP headers that depend on it for some reason so we can't break it.

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