FreeBSD no longer uses GCC as the default compiler. Clang, the new compiler, will not build a working lisp loader binary. GCC must be used instead. I built with GCC 6 but literally any version should work.
Make sure that after you install your GCC you add a link
cd /usr/local/binln -s gcc6 gcc
Make sure 32-bit compatibility libraries are installed (i.e. you have a /usr/lib32 directory). Make sure you've appropriately invoked ldconfig. This is necessary to RUN cmucl in FreeBSD's 64-bit environment at this point.
If you forgot to install the 32-bit libraries when you installed, you can do it manually.
cd /tar xvf <path to lib32.tgz on the install media>
Or you can fetch lib32.tgz from the FreeBSD web site: