[llvm] [Clang] Support target attr specifying CPU (PR #68678)

[Fangrui Song via llvm-commits]

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@@ -2420,11 +2420,11 @@ command line.
 
 The current set of options correspond to the existing "subtarget features" for
 the target with or without a "-mno-" in front corresponding to the absence
-of the feature, as well as ``arch="CPU"`` which will change the default "CPU"
-for the function.
+of the feature, as well as ``arch="CPU"`` and ``cpu="CPU"`` which will change
+the default "CPU" for the function.
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MaskRay wrote:

I have some notes at https://maskray.me/blog/2022-08-28-march-mcpu-mtune . It seems that x86 uses `-march=` for what some other architectures (aarch64,arm,riscv,etc) use for `-mcpu=`...

https://github.com/llvm/llvm-project/pull/68678