Adding a simple program to measure speed of dot products (#1041)

On my Mac, the direct Q4_1 product is marginally slower
(~69 vs ~55 us for Q4_0). The SIMD-ified ggml version
is now almost 2X slower (~121 us).

On a Ryzen 7950X CPU, the direct product for Q4_1 quantization
is faster than the AVX2 implementation (~60 vs ~62 us).

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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>