In practice, this is a strictly stronger condition than target != build
as we never have build = target != host. Really, the attribute should
be removed altogether, but for now we make it work for plain libraries,
which do not care about the target platform. In the few cases where the
compilers use this and actually care about the target platform, I'll
manually change them to use `targetPlatform` instead.

The previous commit redefines `stdenv.cross` for the sake of normal
libaries, the most common use-case of that attribute. Some compilers
however relied on the old definition so we have them use
`targetPlatform` instead. This special casing is fine because we
eventually want to remove `stdenv.cross` and use either `hostPlatform`
or `targetPlatform` instead.

Before all overrides were also pruned in the previous stage, now
only gcc and binutils are, because they alone care about about the
target platform. The rest of the overrides don't, so it's better to
preserve them in order to avoid spurious rebuilds.