If you're asking "what's the source code", then it's built using the
same source as the RPM. After going through %setup (i.e. extracting everything and
applying patches), do:
$ cd fpcsrc/
$ make all CPU_TARGET=aarch64 OS_TARGET=linux BINUTILSPREFIX=aarch64-linux-gnu-
If you're asking "where the binary comes from", then I've cross-compiled
it (as described above) on my x86_64 machine.