docs(README): Update GCC version and download URL

Author: donghengqaz

README.md

@@ -36,12 +36,12 @@ Actions to be taken for *ESP8266_RTOS_SDK v3.0* include the following items:
 
 ## Get toolchain
 
-v5.2.0
+v8.4.0
 
-* [Windows](https://dl.espressif.com/dl/xtensa-lx106-elf-win32-1.22.0-100-ge567ec7-5.2.0.zip)
-* [Mac](https://dl.espressif.com/dl/xtensa-lx106-elf-macos-1.22.0-100-ge567ec7-5.2.0.tar.gz)
-* [Linux(64)](https://dl.espressif.com/dl/xtensa-lx106-elf-linux64-1.22.0-100-ge567ec7-5.2.0.tar.gz)
-* [Linux(32)](https://dl.espressif.com/dl/xtensa-lx106-elf-linux32-1.22.0-100-ge567ec7-5.2.0.tar.gz)
+* [Windows](https://dl.espressif.com/dl/xtensa-lx106-elf-gcc8_4_0-esp-2020r3-win32.zip)
+* [Mac](https://dl.espressif.com/dl/xtensa-lx106-elf-gcc8_4_0-esp-2020r3-macos.tar.gz)
+* [Linux(64)](https://dl.espressif.com/dl/xtensa-lx106-elf-gcc8_4_0-esp-2020r3-linux-amd64.tar.gz)
+* [Linux(32)](https://dl.espressif.com/dl/xtensa-lx106-elf-gcc8_4_0-esp-2020r3-linux-i686.tar.gz)
 
 If you are still using old version SDK(< 3.0), please use toolchain v4.8.5, as following: