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

[Yaxun Liu 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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yxsamliu wrote:

this macro and other AMDGPU predefined macros are documented at https://clang.llvm.org/docs/AMDGPUSupport.html

we should not just mention the link and talk about their dependency on specific GPU arch and taking default value in host compilation.

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