Downgrade the gcc version to version 5.5 on Ubuntu 18.04

First check existing gcc version, the default is 7.3 on Ubuntu 18.04

$ gcc --version
gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

Download gcc/g++ 5

sudo apt-get install -y gcc-5
sudo apt-get install -y g++-5

Link gcc/g++ to downgrade

cd /usr/bin
sudo rm gcc
sudo ln -s gcc-5 gcc
sudo rm g++
sudo ln -s g++-5 g++

Check the gcc version again and you can see that it has been downgraded.
```
$ gcc --version
gcc (Ubuntu 5.5.0-12ubuntu1) 5.5.0 20171010
```

Important: To revert above actions

cd /usr/bin
sudo rm gcc
sudo ln -s gcc-7 gcc
sudo rm g++
sudo ln -s g++-7 g++

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