Implement threading for island constraint solving.

Humanoids22 No threads:
 Benchmark Execution Time: 9.475905812s

Humanoids22 with 10 Threads:
 Benchmark Execution Time: 4.871352s

PiperOrigin-RevId: 571214307
Change-Id: I1f4f2c761b4ae6bc8fac1f28c6c695f6c499f339

src/engine/engine_io.c

@@ -1470,7 +1470,9 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
   //------------------------------ clear header
 
   // clear stack pointer
-  d->pstack = 0;
+  if (!d->threadpool) {
+    d->pstack = 0;
+  }
   d->pbase = 0;
 
   // clear arena pointers