Revert^2 "Remove external/libcxx in favor of prebuilt" am: 9681ed1bea

Original change: https://android-review.googlesource.com/c/platform/external/libcxx/+/3073092

Change-Id: I72568ed06c0dc19a67b43997df83f08a0c7e5835
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>