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
diff --git a/cc/config/mips64_device.go b/cc/config/mips64_device.go
index 4e8bf1e..8997893 100644
--- a/cc/config/mips64_device.go
+++ b/cc/config/mips64_device.go
@@ -90,7 +90,11 @@
 )
 
 func init() {
-	android.RegisterArchFeatures(android.Mips64, "mips64r6",
+	android.RegisterArchVariants(android.Mips64,
+		"mips64r2",
+		"mips64r6")
+	android.RegisterArchFeatures(android.Mips64, "rev6")
+	android.RegisterArchVariantFeatures(android.Mips64, "mips64r6",
 		"rev6")
 
 	pctx.StaticVariable("mips64GccVersion", mips64GccVersion)