Hopefully fixes sandboxed build of glibc on aarch64, as discussed on
8bfa9f528c.
Picked from the following cross-trunk evaluation:
http://hydra.nixos.org/eval/1341395 based on nixpkgs
commit bb3ef8a95c9659596b8a34d27881cd30ffea2f9f.
build job: http://hydra.nixos.org/build/50125932
(busybox's hash not changing is not a bug!)