The issue seems to be that V8 recently moved from GYP to GN build engineering.
There does not seem to be a port of depot_tools to FreeBSD required for the GN build yet.
So perhaps the NodeJS GYP build of V8 is the way to go.
If you do manage to get an up to date port of depot_tools, stick with the official V8 GN build, and please share your solution.