ggml : optimize for ppc64le using VSX intrinsics (ggml/784)

* optimize for ppc64le using VSX intrinsics

* 1. code clean up by removing comments about overflow concern.

2. fix typo in suffix of scaling.

* Continue to fix typo in suffix of scaling for QK_K <> 256

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>