Add additional space to store 64-bit hash codes on 32-bit machines

On 32-bit machines, thread local variables are stored in arrays of
32-bit chunks of memory.  But now that we use 64-bit hash codes, we
were getting the hash counter by concatenating two adjacent elements
of the array, giving us unexpected results.

We add a new thread local variable to store these extra 32 bits,
fixing this issue.

M2/Macaulay2/d/expr.d

@@ -27,6 +27,9 @@ threadCounter := 0;
 threadLocal HashCounter := (
     threadCounter = threadCounter + 1;
     hash_t(1000000 + 3 + (threadCounter-1) * 10000 ));
+-- give 32-bit machines enough space to store a 64-bit hash code
+-- TODO: instead, allow 64-bit entries in the array of thread local variables
+threadLocal HashCounterExtraBits := 0;
 
 export nextHash():hash_t := (
      if HashCounter == Ccode(hash_t, "18446744073709551615ull") -- check for integer overflow