Main thread is a regular field of global_State

They were already allocated as a single block, so there is no need
for the global_State to point to its main thread.

lapi.c

@@ -655,7 +655,7 @@ LUA_API int lua_pushthread (lua_State *L) {
   setthvalue(L, s2v(L->top.p), L);
   api_incr_top(L);
   lua_unlock(L);
-  return (G(L)->mainthread == L);
+  return (mainthread(G(L)) == L);
 }