ggml-hexagon: flash-attention and reduce-sum optimizations (#19141)

ggml-hexagon: flash-attention and reduce-sum optimizations (#19141)

* wip

* ggml-hexagon: add vectorized dot product function for FP32 and FP16 accumulation

* ggml-hexagon: optimize dot product functions for FP16 and FP32 with new vectorized implementations

* wip

* ggml-hexagon: optimize hvx_vec_dump_f32_n and hvx_vec_reduce_sum_qf32x2 functions for improved performance

* ggml-hexagon: refactor dot product functions to use a common loading function for improved readability

* optimize vector dot product functions to use unified reduction for improved performance

* wip

* ggml-hexagon: add vectorized dot product function for FP32 and FP16 accumulation

* ggml-hexagon: optimize dot product functions for FP16 and FP32 with new vectorized implementations

* wip

* ggml-hexagon: optimize hvx_vec_dump_f32_n and hvx_vec_reduce_sum_qf32x2 functions for improved performance

* ggml-hexagon: refactor dot product functions to use a common loading function for improved readability

* optimize vector dot product functions to use unified reduction for improved performance

* hexagon: optimize reduce-sum for v75+

* hexagon: always keep row_sums in sf/fp32

* ggml-hexagon: enhance directory checks for HEXAGON_SDK_ROOT and HEXAGON_TOOLS_ROOT

* fix compiling error after rebase

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Co-authored-by: Max Krasnyansky <redacted>