Upgrade GCC to 7.3.0

Download GCC Source
https://mirrors.tuna.tsinghua.edu.cn/gnu/gcc/gcc-7.3.0/gcc-7.3.0.tar.gz
or find a proper release in here: https://gcc.gnu.org/releases.html

Install GCC

% tar xvf gcc-7.3.0.tar.gz
% cd gcc-7.3.0
% mkdir build && cd build
% ../configure --prefix=/usr --enable-languages=c,c++ --disable-multilib
% make
% sudo make install
% gcc --version