Multi-thread Performance Improvement of GEMM with DIVIDE_RATE=1 for A64FX

[nakagawa-fj]

Closes #5347
This PR improves the multi-thread performance of GEMM on A64FX by setting DIVIDE_RATE to 1.
The thread control in GEMM currently uses default value of DIVIDE_RATE=2, which always splits N dimension of matrix into two parts for computation. However, this splitting occurs even when N is small (e.g., N=2), leading to a decrease in computational efficiency.
For GEMM on A64FX, I tried DIVIDE_RATE=1 and confirmed performance improvements as shown in the graphs below.
While improvements were expected for narrow matrices with small N dimensions, performance gains were also observed for square matrices.

[martin-frbg]

That's a very insteresting result - somewhat counterintuitive, but I guess if it helps to make optimum use of the vector length... Certainly a reminder that some core design decisions for OpenBLAS (GotoBLAS really) were made on and for the cpu architectures of some twenty years ago.
I notice that DIVIDE_RATE is also used in the multithreaded SYRK code (and GETRF/POTRF, though they may be speed-limited by other constraints). And I guess it will be interesting to see if a non-default value would also be beneficial for other (SVE/SME) arm64 or RISCV-vector platforms