[docs] fix outdated GNU GCC toolchain link in qemu-vexpress-a9 README

The link points to launchpad.net, which shut down its gcc-arm-embedded
downloads years ago, and the tarball it names is a 2016 Q3 build
(gcc 5.4). Point to ARM's official GNU-RM download page instead, and
mention that the distribution toolchain works too, since rtconfig.py
already defaults EXEC_PATH to /usr/bin.

Author: Cheng Li

bsp/qemu-vexpress-a9/README.md

@@ -40,7 +40,7 @@
 
   ![image-20220626114020634](figures/image-20220626114020634.png)
 
-- 如果在Linux下使用，请自行下载[GNU GCC工具链][3]。
+- 如果在Linux下使用，可以直接使用发行版软件源安装的工具链：`sudo apt install gcc-arm-none-eabi binutils-arm-none-eabi libstdc++-arm-none-eabi-newlib`（rtconfig.py 默认的 `EXEC_PATH` 即为 `/usr/bin`）。也可以从 [ARM 官网][3] 下载 GNU Arm Embedded Toolchain 自行安装。
 
 ## 3 执行和退出
 
@@ -244,6 +244,6 @@ msh />QEMU: Terminated
 
 [1]: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.subset.boards.express/index.html
 [2]: https://www.rt-thread.org/download.html#download-rt-thread-env-tool
-[3]: https://launchpad.net/gcc-arm-embedded/5.0/5-2016-q3-update/+download/gcc-arm-none-eabi-5_4-2016q3-20160926-linux.tar.bz2
+[3]: https://developer.arm.com/downloads/-/gnu-rm
 [4]: https://github.com/BernardXiong
 [5]: https://github.com/recan-li