Add aarch64-android testing

In updating our image in rust-lang#120326 we were forced off the 32-bit image.
This means it is not too difficult to add testing for 64-bit code as
well. Since this architecture is more used, and on later images, the
only supported kind of executable, we should start testing this now.

src/ci/docker/host-x86_64/aarch64-android/Dockerfile

@@ -28,7 +28,7 @@ ENV PATH=$PATH:/android/sdk/emulator
 ENV PATH=$PATH:/android/sdk/tools
 ENV PATH=$PATH:/android/sdk/platform-tools
 
-ENV TARGETS=arm-linux-androideabi
+ENV TARGETS=arm-linux-androideabi,aarch64-linux-android
 
 ENV RUST_CONFIGURE_ARGS --android-ndk=/android/ndk/