image: add platform support for s390x, ppc64le in the ISO

This commit populates the "img.Platform" struct when building
ISOs for s390x and ppc64le.

No tests right now sadly, this should be looked into but given
that it is known broken right now this change can hardly make
it worse.

Closes: #686

bib/cmd/bootc-image-builder/image.go

@@ -425,6 +425,22 @@ func manifestForISO(c *ManifestConfig, rng *rand.Rand) (*manifest.Manifest, erro
 			},
 			UEFIVendor: c.SourceInfo.UEFIVendor,
 		}
+	case arch.ARCH_S390X:
+		img.Platform = &platform.S390X{
+			Zipl: true,
+			BasePlatform: platform.BasePlatform{
+				ImageFormat: platform.FORMAT_ISO,
+			},
+		}
+	case arch.ARCH_PPC64LE:
+		img.Platform = &platform.PPC64LE{
+			BIOS: true,
+			BasePlatform: platform.BasePlatform{
+				ImageFormat: platform.FORMAT_ISO,
+			},
+		}
+	default:
+		return nil, fmt.Errorf("unsupported architecture %v", c.Architecture)
 	}
 
 	img.Filename = "install.iso"