make 64-bit builds into multiarch builds

Also enable the 64-bit binder interface and make them only use ART.

Change-Id: I05de41ce348c14722c84a658f745659bde5ba423
diff --git a/target/product/aosp_arm64.mk b/target/product/aosp_arm64.mk
index 7f97a61..8ff0bd1 100644
--- a/target/product/aosp_arm64.mk
+++ b/target/product/aosp_arm64.mk
@@ -22,6 +22,8 @@
 $(call inherit-product, $(SRC_TARGET_DIR)/product/aosp_base_telephony.mk)
 $(call inherit-product, $(SRC_TARGET_DIR)/board/generic_arm64/device.mk)
 
+PRODUCT_RUNTIMES := runtime_libart_default
+
 include $(SRC_TARGET_DIR)/product/emulator.mk
 PRODUCT_NAME := aosp_arm64
 PRODUCT_DEVICE := generic_arm64