These prebuilts were built automatically on the Fuchsia build infrastructure.
The source used to generate these objects can be found at https://fuchsia.googlesource.com/third_party/gcc_none_toolchains at the SHA that is the package filename. The SHA will also appended to the end of this file.
These tools are built by a combination of upstream sources and patches. The patches can be found in the gcc_none_toolchains repo and the upstream sources have been mirrored to Google Storage for posterity.
List all the upstream sources: gsutil ls gs://fuchsia-build/zircon/toolchain/sources/
You can download them with HTTPS at addresses with the form of: https://fuchsia-build.storage.googleapis.com/zircon/toolchain/sources/[package name] (E.g. https://fuchsia-build.storage.googleapis.com/zircon/toolchain/sources/gcc-5.3.0.tar.bz2)