Some news from OpenBSD 76 front.
I installed a fresh release of OpenBSD 76, from scratch (and so not a upgrade from 75).
The binary from there:
https://downloads.freepascal.org/fpc/snapshot/fixes_3_2/x86_64-openbsd/ (thanks to BSaidus) can compile and the apps are running.
Only needed to create synlink from the needed libc.so.86.0 and libpthread,so.57.0 (not sure by memory about the version) in fpc to the installed version in OpenBSD 76.
So to resume, it seems that OpenBSD 75 and his upgrade is not for fpc. (but OpenBSD 74 is ok)
But for compiling fpc himself on OpenBSD 76, using last fpupdeluxe or from gmake makefile from fpc, the compilation fails.
It would be interesting to know how those binaries were compiled from
https://downloads.freepascal.org/fpc/snapshot.