The "37.10.5. Compiling and Linking Dynamically-Loaded Functions" (https://www.postgresql.org/docs/devel/xfunc-c.html#DFUNC) mentions outdated compiler for FreeBSD (gcc). As of FreeBSD 13.0 and going forward, all supported architectures abandoned gcc and use LLVM/clang. The FreeBSD 13.0 is already EOL, therefore all supported FreeBSD releases for all architectures have clang as the default compiler. It is still allowed to use gcc but it's not installed by default.
"Removed the obsolete binutils 2.17 and gcc(1) 4.2.1 from the tree. All supported architectures now use the LLVM/clang toolchain. 0ad202f312f6, a04ec978b369, 57f804675e65, 90b9aa475e9e (Sponsored by The FreeBSD Foundation)"
Therefore please update compilation instructions for FreeBSD for future PostgreSQL versions by replacing "gcc" with "cc" and changing version information from FreeBSD 3.0 to 13.0.