top-level: Lay the groundwork for `{build,host,target}Platform`

The long term goal is a big replace:
  { inherit system platform; } => buildPlatform
  crossSystem => hostPlatform
  stdenv.cross => targetPlatform
And additionally making sure each is defined even when not cross compiling.

This commit refactors the bootstrapping code along that vision, but leaves
the old identifiers with their null semantics in place so packages can be
modernized incrementally.