Fix the integer overflow issue for large matrix size

For large matrix, e.g. M=N=K, and M>1290, int mnk=M*N*K will overflow.
This will lead to wrong branching to single-threading. The performance
is downgraded significantly.

Signed-off-by: Wang, Long <long1.wang@intel.com>

kernel/x86_64/sgemm_kernel_16x4_skylakex.c

@@ -1215,7 +1215,7 @@ CNAME(BLASLONG m, BLASLONG n, BLASLONG k, float alpha, float * __restrict A, flo
 
 int sgemm_kernel_direct_performant(BLASLONG M, BLASLONG N, BLASLONG K)
 {
-	int mnk = M * N * K;
+	unsigned long mnk = M * N * K;
 	/* large matrixes -> not performant */
 	if (mnk >= 28 * 512 * 512)
 		return 0;

kernel/x86_64/sgemm_kernel_16x4_skylakex_2.c

@@ -452,7 +452,7 @@ CNAME(BLASLONG m, BLASLONG n, BLASLONG k, float alpha, float * __restrict__ A, f
 
 int sgemm_kernel_direct_performant(BLASLONG M, BLASLONG N, BLASLONG K)
 {
-	int mnk = M * N * K;
+	unsigned long mnk = M * N * K;
 	/* large matrixes -> not performant */
 	if (mnk >= 28 * 512 * 512)
 		return 0;