Add prepareClone and atClone APIs for Verilated models (#3503) (#4444)

This API is used if the user copies the process using `fork`
and similar OS-level mechanisms. The `at_clone` member function
ensures that all model-allocated resources are re-allocated, such
that the copied child process/model can simulate correctly.

A typical allocated resource is the thread pool, which every model
has its own pool.

src/V3EmitCSyms.cpp

@@ -468,7 +468,7 @@ void EmitCSyms::emitSymHdr() {
 
     if (v3Global.opt.mtasks()) {
         puts("\n// MULTI-THREADING\n");
-        puts("VlThreadPool* const __Vm_threadPoolp;\n");
+        puts("VlThreadPool* __Vm_threadPoolp;\n");
         puts("bool __Vm_even_cycle__ico = false;\n");
         puts("bool __Vm_even_cycle__act = false;\n");
         puts("bool __Vm_even_cycle__nba = false;\n");