Ability to generate SDK system-images for arm64 and x86_64

(Cherry-picked from 40c7126e7cfe4680cadd11b90774f9996a8e2d37)

Change-Id: I2715a668271ed88fcd0c8c97ac35d05fcdd34a06
diff --git a/sdk/images_x86_source.prop_template b/sdk/images_x86_source.prop_template
index cfbe180..1a18cf8 100644
--- a/sdk/images_x86_source.prop_template
+++ b/sdk/images_x86_source.prop_template
@@ -3,7 +3,7 @@
 Pkg.Revision=1
 AndroidVersion.ApiLevel=${PLATFORM_SDK_VERSION}
 AndroidVersion.CodeName=${PLATFORM_VERSION_CODENAME}
-SystemImage.Abi=x86
+SystemImage.Abi=${TARGET_CPU_ABI}
 SystemImage.GpuSupport=true
 SystemImage.TagId=default