This has nothing to do with sysmerge(8). That program is for merging changes to files in /etc and /var.
Your problem was caused by a "library bump." In this case, the C library had a major bump,
from 68.4 to 69.0, due to
asr symbol changes.
Briefly, when Xenocara was built (on June 9), the C library was still at 68.4. You upgraded from a system with an even earlier library, so you do not have 68.4 in your /usr/lib. A fresh install would have the same issue.
The library bump occurred 34 hours ago at this writing. I expect a Xenocara build to follow soon. If you are impatient, you can build it on a modern i386 machine in about an hour, if you have the source code (/usr/xenocara by default). See the man page I referenced in my post above (step 5) and also FAQ 5 for guidance on building Xenocara, if you are interested.