Revert "stdenv: aarch64 is also ARM"

This reverts commit a5f4e222.

Breaks the stdenv build, e.g.: http://hydra.nixos.org/build/50015717
In general, the architectures are different enough that there is no
reason to consider both as ARM, just like we don't consider x86_64 as
32-bit x86.

cc @fpletz