Dynamically generate arch struct

Now we don't need to hardcode the list of OS/Arch/Variant/Features in
android/arch.go.

Change-Id: I0f9cc35d55baa31f036825fdf5b9dd30d076e56e

cc/config/arm64_device.go

@@ -89,6 +89,11 @@ const (
 )
 
 func init() {
+	android.RegisterArchVariants(android.Arm64,
+		"armv8_a",
+		"cortex_a53",
+		"denver64")
+
 	pctx.StaticVariable("arm64GccVersion", arm64GccVersion)
 
 	pctx.SourcePathVariable("Arm64GccRoot",