Revert "Remove external/libcxx in favor of prebuilt" am: 45e6c488a5 am: ae27b2e5ff

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

Change-Id: Iaa2535b4d51415a18a1f27ca682d07f8f7af290a
Signed-off-by: Automerger Merge Worker <[email protected]>