Add "cros" subarch for ChromeOS verified boot

The default partition table when formatting disks is decided in the partman-partitioning package based on arch/subarch detected here. Non-EFI x86 systems default to formatting disks as MBR, but the ChromeOS bootloader only supports GPT. To default to GPT with these devices, this MR detects them as a "cros" subarch on applicable architectures (x86, arm and arm64), based on the cros_secure kernel command line argument that the ChromeOS bootloader prepends. (Also see partman-partitioning!4).