Now that I'm back home, I checked and yes, I can build the cross compiler on macOS, I just cannot build the required FreeBSD units. That is:
ls -l usr/local/share/fpc-3.1.1/rtl/units/
drwxr-xr-x 200 root wheel 6400 10 Jul 16:31 x86_64-darwin <<-- OK
drwxr-xr-x 2 root wheel 64 10 Jul 16:31 x86_64-freebsd <<-- EMPTY :(
The problem is:
make[5]: x86_64-freebsd-as: No such file or directory
which seems to suggest to me that macOS is looking for a FreeBSD x86_64 (cross?) assembler that does not exist.