gcc toolchain BUILD.go served from a .tar.gz on storage.googleapis.com

[paulhankin]

We serve x86_64-elf-gcc from storage.googleapis.com. That's fine for the binaries (and I guess something like this is necessary to avoid the push limits of github).

But we also have BUILD.go in there, which specifies the toolchain.

I feel like that file should be in version control (ie: github). It's unlikely we want to edit gcc source files, but we do want to occasionally edit the BUILD.go. Right now apart from being a bit of a pain, the process for changing BUILD.go is very unclear.

Could we have a github repo per toolchain with just the BUILD.go file, and a dependency on the binaries on storage.googleapis?
This would also make specifying a version of the toolchain easier as we can depend on git branches and tags like for other versioned things.