The only supported way to bootstrap FPC always has been and always will be based on a complete installation of the previous official release. The bootstrap compilers are sometimes uploaded for certain source-centric packagers, but they are 100% unsupported and you should not use them.
If it were up to me, all of these bootstrap compilers would be removed from the ftp site, because using them instead of a full installation can lead under some circumstances to hard to diagnose problems if you don't know the details of the FPC build process.