Revert "Remove external/libcxx in favor of prebuilt" am: 45e6c488a5

Original change: https://android-review.googlesource.com/c/platform/external/libcxx/+/3061869

Change-Id: I10cf37d1224984e6aa248a07b31ffdb481798c86
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>