fixup! support for the Genode OS framework (#5653)

Allocate thread metadata at createThread.

lib/genode_cpp/threads.h

@@ -13,7 +13,6 @@
 #define _GENODE_CPP__THREAD_H_
 
 #include <base/thread.h>
-#include <util/avl_tree.h>
 #include <util/reconstructible.h>
 
 namespace Nim { struct SysThread; }

lib/system/threads.nim

@@ -593,6 +593,8 @@ elif defined(genode):
   proc createThread*[TArg](t: var Thread[TArg],
                            tp: proc (arg: TArg) {.thread, nimcall.},
                            param: TArg) =
+    t.core = cast[PGcThread](allocShared0(sizeof(GcThread)))
+
     when TArg isnot void: t.data = param
     t.dataFn = tp
     when hasSharedHeap: t.stackSize = ThreadStackSize