Refine SVE2 Gemm32fNT kernels with zero init and Kernel6x4nt A/B order

Initialize accumulators from const svfloat32_t zero = svdup_n_f32(0.0f).
In Kernel6x4nt, load B rows first and use svmla(c, b, a) to reduce
register pressure versus holding six A vectors.

Co-authored-by: Ihar Yermalayeu <ermig1979@gmail.com>

Author: cursoragent