give snapshot load threads semi-lisp status

they grab mutexes and allocate. the former more or less requires
a thread, and the latter definitely does with whippet or mps.
These threads shouldn't need a VM however.

include/clasp/gctools/threadlocal.h

@@ -109,14 +109,14 @@ struct VirtualMachine {
   // grow and etc.
   static constexpr size_t MaxStackWords = 65536;
   bool _Running;
-  core::T_O** _stackBottom;
+  core::T_O** _stackBottom = nullptr;
   size_t _stackBytes;
   core::T_O** _stackTop;
   core::T_O** _stackGuard;
   core::T_O** _stackPointer;
   // only used by debugger
   // has to be initialized because bytecode_call reads it
-  core::T_O** _framePointer = NULL;
+  core::T_O** _framePointer = nullptr;
 #ifdef DEBUG_VIRTUAL_MACHINE
   core::T_O* _data;
   core::T_O* _data1;

src/gctools/snapshotSaveLoad.cc

@@ -3474,6 +3474,12 @@ void snapshot_load(void* maybeStartOfSnapshot, void* maybeEndOfSnapshot, const s
               // lookup will cause multicore linking and with multicore linking we have to do things after this in a thread safe way
               size_t objectId = allocatedObjectFile->_ObjectId;
               pool.push_task([&obj_claspJIT, jitdylib, objectId]() {
+                // do_lookup can allocate, so set up a bit of a Lisp thread.
+                void* stacktop = &stacktop;
+                gctools::ThreadLocalStateLowLevel tlsll(stacktop);
+                core::ThreadLocalState tls;
+                my_thread_low_level = &tlsll;
+                my_thread = &tls;
                 std::string start;
                 std::string shutdown;
                 core::startup_shutdown_names(objectId, "", start, shutdown);

src/gctools/threadlocal.cc

@@ -171,7 +171,10 @@ VirtualMachine::~VirtualMachine() {
 #if 1
   this->disable_guards();
 #endif
-  gctools::RootClassAllocator<T_O>::freeRoots(this->_stackBottom);
+  // In snapshot load we make threads with no VM.
+  // We have nothing to free and _stackBottom is just its initial NULL.
+  if (this->_stackBottom)
+    gctools::RootClassAllocator<T_O>::freeRoots(this->_stackBottom);
 }
 
 // For main thread initialization - it happens too early and _Nil is undefined