Gate macOS-only subcommands and arguments to macOS

[stackotter]

At the moment commands such as list-identities and simulators are still available on Linux even though they don't work on Linux. If they ever work on Linux in the future we can add them back.

Simply disabling the subcommands on Linux is easy enough and not necessarily issue worthy, but I think this is a good opportunity to architect how Swift Bundler will handle host-platform specific bundler features (at the moment it only has to deal with target-platform specific features cause it mostly assumes that it's running on macOS).