- Dispatch off more appropriate conditions---`stdenv.cc.is*` and
`hostPlatform.is*` directly---rather than the OS as a proxy.
- Don't worry about pulling in binutils from normal `stdenv.cc` for
`gccMultiStdenv`.
- Defining a `multiStdenv` that uses whatever compiler is default.
- Define `stdenv_32bit` in terms of `multiStdenv`.