- only skip `--target` for proc-macros
- don't try build non-host targets for proc-macros, since they're not
  supported anyway.
- change rustwide_builder to look in the platform-specific target
  directory for the default target
- stores all source docs in the target/{target_name} directory, rather
  than trying to special case anything
  - this also creates the `{target_name}` directory before copying,
    because `rename` delegates directly to the syscall and doesn't
    create parent directories