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/distro/rhel9/distro.go

@@ -293,6 +293,17 @@ func newDistro(name string, major, minor int) distro.Distro {
 	x86_64.addImageTypes(
 		rawX86Platform,
 		mkAMIImgTypeX86_64(rd.osVersion, rd.isRHEL()),
+	)
+
+	gceX86Platform := &platform.X86{
+		BIOS:       true,
+		UEFIVendor: rd.vendor,
+		BasePlatform: platform.BasePlatform{
+			ImageFormat: platform.FORMAT_GCE,
+		},
+	}
+	x86_64.addImageTypes(
+		gceX86Platform,
 		gceImgType,
 	)
 
@@ -443,7 +454,7 @@ func newDistro(name string, major, minor int) distro.Distro {
 		)
 
 		// add GCE RHUI image to RHEL only
-		x86_64.addImageTypes(rawX86Platform, gceRhuiImgType)
+		x86_64.addImageTypes(gceX86Platform, gceRhuiImgType)
 	} else {
 		x86_64.addImageTypes(azureX64Platform, azureImgType)
 	}

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")
 	}