I can't submit this in smaller units because the various components all
depend on one another during the stdenv bootstrap, so I think this is
the smallest sensible change I can make.
I also removed the symbol-hiding shenanigans in Libsystem. It might mess
up compatibility with 10.9 but I don't really want to support the added
complexity and I see little evidence of anyone else wanting to support
it. If someone cares, we might be able to revive compatibility, but for
now it'll stay like this.
948b7f23bb