Rewrite a few `stdenv.cross` uses that *should* be `targetPlatform`

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.