We can use use `stdenv.hostPlatform.isStatic` instead, and move the
logic per package. The least opionated benefit of this is that it makes
it much easier to replace packages with modified ones, as there is no
longer any issue of overlay order.
CC @FRidh @matthewbauer
These shouldn't respond to targetPlatform, but previously did. The
reason is somewhat complex: they would rely on the sources of gcc and
binutils, respectively, which *do* depend on the target platform.
Obviously the source is the same in all cases, but when those packages
are no longer preserved from bootstrapping stages their `src` attributes
use a different fetchurl resulting in a changed hash.
