Move stdenv bootstrap binaries out of the tree

Commit 986f3619 started to use
<nix/fetchurl.nix> to "download" the bootstrap binaries from the
Nixpkgs tree, using the file:/// scheme. This has really bad
consequences:

* It makes any derivation depend on the path of the Nixpkgs tree. So
  evaluating a package will produce a different .drv file when run
  from different locations. No wonder Hydra evaluation has been so
  slow lately: for every Nixpkgs evaluation, it had to create tens of
  thousands of .drv files, even if nothing had changed.

* It requires the builder to have file system access to the Nixpkgs
  tree. So if your tree is in your home directory, the stdenv
  bootstrap would probably fail.

So now the binaries are downloaded from tarballs.nixos.org.

Also dropped PowerPC "support".