jl_rng_split: permute state by multiply instead of add

Issue raised here:

https://discourse.julialang.org/t/linear-relationship-between-xoshiro-tasks/110454

src/task.c

@@ -1079,7 +1079,7 @@ void jl_rng_split(uint64_t dst[JL_RNG_SIZE], uint64_t src[JL_RNG_SIZE]) JL_NOTSA
         p ^= p >> ((p >> 59) + 5);
         p *= m[i];
         p ^= p >> 43;
-        dst[i] = src[i] + p; // SplitMix dot product
+        dst[i] = src[i] * (p | 1); // multiplicative permutation
     }
 }