If you built userland properly, all the object files are in /usr/obj, which can also be cleared completely:
# rm -rf /usr/obj/*
Some small savings can also be had by clearing your kernel build of object files also. Here's an example with GENERIC,
if you built GENERIC.MP or you have custom kernels, do the same there:
# cd /usr/src/sys/arch/<arch>/compile/GENERIC
# make clean
|