freertos: Add SPDX license identifiers and update xtensa port files

This commit does the following:

- Adds SPDX license identifiers to FreeRTOS sources. Remove those FreeRTOS sources from
  the copyright ignore list.
- Update xtensa port files to match FreeRTOS v10.4.3. Added SPDX license identifiers
  to the port files.
- Fixed some improperly licensed files
- Removed portbenchmark.h from RISC-V port

components/freertos/FreeRTOS-Kernel/stream_buffer.c

@@ -1,3 +1,10 @@
+/*
+ * SPDX-FileCopyrightText: 2020 Amazon.com, Inc. or its affiliates
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * SPDX-FileContributor: 2016-2022 Espressif Systems (Shanghai) CO LTD
+ */
 /*
  * FreeRTOS Kernel V10.4.3
  * Copyright (C) 2020 Amazon.com, Inc. or its affiliates.  All Rights Reserved.