I've made a quick transcription here, but future readers should be warned it may have become out-of-date.
How to build the toolchain from sources
You can build Arm GNU Toolchain from sources using Linaro ABE (Advanced Build Environment) and provided ABE manifest files.
Below example shows how to build gcc-arm-aarch64-none-elf toolchain from sources using Linaro ABE build system.
Instructions
ABE has a dependency on git-new-workdir and needs this tool to be installed in /usr/local/bin directory:
$ wget https://raw.githubusercontent.com/git/git/master/contrib/workdir/git-new-workdir
$ sudo mv git-new-workdir /usr/local/bin
$ sudo chmod +x /usr/local/bin/git-new-workdir
Clone ABE from the URL below and checkout the stablebranch (see Getting ABE):
$ git clone https://git.linaro.org/toolchain/abe.git
Create the build directory and change to it. Any name for the directory will work:
$ mkdir build && cd build
Configure ABE (from the build directory):
$ ../abe/configure
Download the toolchain manifest file, from https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/downloads, into the build folder, for the required toolchain, for example, gcc-arm-aarch64-none-elf-abe-manifest.txt:
$ wget https://developer.arm.com/-/media/Files/downloads/gnu/11.2-2022.02/manifest/gcc-arm-aarch64-none-elf-abe-manifest.txt
Build toolchain (from the build directory):
$ ../abe/abe.sh --manifest gcc-arm-aarch64-none-elf-abe-manifest.txt --build all
The built toolchain will be installed and available for use in the builds/destdir/x86_64-unknown-linux-gnu/bin/ directory.