cmake : fix compile assumptions for power9/etc #2777
                
     Merged
            
            
          
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A regression was introduced in the move to CMake and with the
patch for POWER10 support.
This patch corrects CMake build logic to correctly detect and define
-cpu=power9 during build, which turns back on all the VSX codepaths
for POWER9 machines.
Results especially with -DGGML_BLAS=1 restore whisper.cpp performance
to faster-than-realtime transcription with the medium-q5 model on a
Talos II underclocked to 3.3GHz.