newlib: rebuild with the global stdio streams enabled.

This change means that there is only one set of global stdin, stdout, and stderr
FILE streams shared by all the threads. This reduces memory usage and avoids
having to close these streams before threads exit. These streams still have a
lock to synchronise access.

core/app_main.c

@@ -431,8 +431,6 @@ static __attribute__((noinline)) void user_start_phase2(void) {
         memcpy(&phy_info, &default_phy_info, sizeof(sdk_phy_info_t));
     }
 
-    // Disable default buffering on stdout
-    setbuf(stdout, NULL);
     // Wait for UARTs to finish sending anything in their queues.
     uart_flush_txfifo(0);
     uart_flush_txfifo(1);