Revert^2 "Remove external/libcxx in favor of prebuilt" am: 9681ed1bea am: 280cb34cc2

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

Change-Id: I5d7dab7e630c02c3dbe7b62706e75accba02d99e
Signed-off-by: Automerger Merge Worker <[email protected]>