New control for reentrancy of emergency collections

Instead of assuming that shrinking a block may be an emergency
collection, use an explicit field ('gcstopem') to stop emergency
collections while GC is working.

lstate.c

@@ -379,6 +379,7 @@ LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) {
   g->panic = NULL;
   g->gcstate = GCSpause;
   g->gckind = KGC_INC;
+  g->gcstopem = 0;
   g->gcemergency = 0;
   g->finobj = g->tobefnz = g->fixedgc = NULL;
   g->firstold1 = g->survival = g->old1 = g->reallyold = NULL;