Tested under debian 9.3.0 stretch.
Building cross compilers requires gcc & binutils sources.
Download https://ftp.gnu.org/gnu/binutils/binutils-2.30.tar.gz and extract into $HOME/src/binutils-2.30 folder.
Download https://ftp.gnu.org/gnu/gcc/gcc-7.3.0/gcc-7.3.0.tar.gz and extract into $HOME/src/gcc-7.3.0 folder.