The SVN (and the GIT one also) repositories are the same for all architectures, support for different architectures is abstracted under the /usr/src/sys/<architecture> directories.
The stable branch for FreeBSD 11 will be called stable/11 in the SVN repository. It will be created by making full copy (with copy-on-write semantics) of head, the exact operation will be a simple svn copy; svn commit by a committer. Once the stable branch is created the pre-release cycle for 11.0-RELEASE starts in it and whatever fixes it requires will be commited to stable/11. After that the releng/11.0 branch is created, this will be a svn copy of the stable/11 branch. At this point the release candidates are created from releng/11.0 and last fixes are committed before the real 11.0-RELEASE is released.
After all of this head will be bumped up to FreeBSD 12 and it will start its own development cycle for those releases. This will be initially just a simple edit in /usr/src/sys/conf/newvers.sh to change the version number.
I don't see how the 32-bit Intel platform would be pushed back in tiers anytime soon, it's still the most used platform in the world and there's no end in sign of its usability to wide variety of applications.
