Upcoming change to build config

This upcoming change will change the run_mksnapshot build target to produce one more file. The background is that we want to make builtin code objects isolate-independent and move them off V8's heap and embed them in the binary instead.

I don't think this will cause breakages in the short term, since this configuration is gated by a build flag. It makes sense to look out for this and maybe port to gyp though.