platform: add special GCE platform format

GCE requires an archive with specific (tar) options, so we should have a
platform format defined separately for it.
Add the GCE image types to the x86_64 arch using the new platform
format.

internal/platform/platform.go

@@ -17,6 +17,7 @@ const (
 	FORMAT_QCOW2
 	FORMAT_VMDK
 	FORMAT_VHD
+	FORMAT_GCE
 )
 
 func (a Arch) String() string {
@@ -46,6 +47,8 @@ func (f ImageFormat) String() string {
 		return "vmdk"
 	case FORMAT_VHD:
 		return "vhd"
+	case FORMAT_GCE:
+		return "gce"
 	default:
 		panic("invalid image format")
 	}