loongarch64: fix segfaults in copy kernels and adjust dsyrk block size

[HecaiYuan]

On LA464, dsyrk produces incorrect results with specific block dimension combinations (e.g., 80×16), which causes dcholesky and dpotrf to fail on positive-definite matrices.

Reduce GEMM_P from 112 to 96 for LoongArch64 LA464 double-precision builds, so the blocking logic avoids generating the problematic size.

The following minimal C program was used to pinpoint the issue:

// test_dsyrk.c
#include <stdio.h>
#include <cblas.h>
#include <stdlib.h>

#define N 194
int main(){
    double *A = malloc(N*N*sizeof(double));
    double *B = malloc(N*N*sizeof(double));
    for(int j=0;j<N;j++){
        for(int i=0;i<j;i++) A[i + j*N] = (double)rand()/RAND_MAX - 0.5;
        A[j + j*N] = (double)rand()/RAND_MAX + 8.0;
        for(int i=j+1;i<N;i++) A[i + j*N] = 0.0;
    }

    double alpha = 1.0, beta = 0.0;
    cblas_dsyrk(CblasColMajor, CblasLower, CblasNoTrans, N, N, alpha, A, N, beta, B, N);

    for (int i = 0; i < N; i++) 
        printf("B[%d][%d] = %g\n", i, i, B[i + i*N]);

    free(A); free(B);
    return 0;
}

Compile and run:

gcc test_dsyrk.c -o testd_syrk -I. -L. -lopenblas -lpthread -lm
./test_dsyrk

After fix: all diagonal elements remain positive as expected.