The logic was made pure for the normal libSystem, but this change never
made it to the bootstrap tools. Deduplication the logic as the comment
suggests would have prevented this, but here's a stop-gap until we do
so.
This reinstates the libSystem selective symbol export machinery we used
to have, but locks it to the symbols that were present in 10.11 and skips
the actual compiled code we put into that library in favor of the system
initialization code. That should make it more stable and less likely to
do weird stuff than the last time we did this.
