libc
This is especially useful when not cross compiling. It means we can remove the `stdenv.isGlibc` predicate too. Additionally, use this to simplify the logic to choose the appropriate libiconv derivation.
`hostPlatform.isDarwin` instead of `lib.system.parse.isDarwin hostPlatform.parsed`
localSystem
crossSystem
Previously, platforms was a random thing in top-level
