Replace mul in write_matrix.s

Refactor `write_matrix.s` to avoid using the `mul` instruction by
implementing a custom solution for multiplication. This approach
is inspired by the cumulative addition/subtraction logic used in
the earlier `dot` operation implementation.

Negative numbers are not handled here, as both the number of rows
and columns are guaranteed to be positive.

Author: yy214123

src/write_matrix.s

@@ -63,7 +63,15 @@ write_matrix:
 
     # mul s4, s2, s3   # s4 = total elements
     # FIXME: Replace 'mul' with your own implementation
-
+    li t5, 0
+    
+accumulate:
+    add t5,t5,s2
+    addi s3, s3, -1
+    bne s3, zero, accumulate
+    
+    mv s4, t5
+    
     # write matrix data to file
     mv a0, s0
     mv a1, s1        # matrix data pointer