Correct handling of 'luaV_execute' invocations

The previous stackless implementations marked all 'luaV_execute'
invocations as fresh. However, re-entering 'luaV_execute' when
resuming a coroutine should not be a fresh invocation. (It works
because 'unroll' called 'luaV_execute' for each call entry, but
it was slower than letting 'luaV_execute' finish all non-fresh
invocations.)

lstate.c

@@ -172,7 +172,7 @@ void luaE_checkcstack (lua_State *L) {
 
 LUAI_FUNC void luaE_incCstack (lua_State *L) {
   L->nCcalls++;
-  if (getCcalls(L) >= LUAI_MAXCCALLS)
+  if (unlikely(getCcalls(L) >= LUAI_MAXCCALLS))
     luaE_checkcstack(L);
 }