Threads are created like other objects

Using a version of 'luaC_newobj' that allows offsets (extra space
before the object itself).

lgc.h

@@ -190,6 +190,8 @@ LUAI_FUNC void luaC_step (lua_State *L);
 LUAI_FUNC void luaC_runtilstate (lua_State *L, int statesmask);
 LUAI_FUNC void luaC_fullgc (lua_State *L, int isemergency);
 LUAI_FUNC GCObject *luaC_newobj (lua_State *L, int tt, size_t sz);
+LUAI_FUNC GCObject *luaC_newobjdt (lua_State *L, int tt, size_t sz,
+                                                 size_t offset);
 LUAI_FUNC void luaC_barrier_ (lua_State *L, GCObject *o, GCObject *v);
 LUAI_FUNC void luaC_barrierback_ (lua_State *L, GCObject *o);
 LUAI_FUNC void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt);