ports: Call gc_sweep_all() when doing a soft reset.

This calls finalisers of things like files and sockets to cleanly close
them.

ports/esp8266/main.c

@@ -88,6 +88,7 @@ STATIC void mp_reset(void) {
 }
 
 void soft_reset(void) {
+    gc_sweep_all();
     mp_hal_stdout_tx_str("PYB: soft reboot\r\n");
     mp_hal_delay_us(10000); // allow UART to flush output
     mp_reset();