Build sdk variants of libc++ for riscv64 am: ecb2748e58

Original change: https://android-review.googlesource.com/c/platform/external/libcxxabi/+/2405115

Change-Id: I76a1f72c08fa44e4966412fc80d5957d7dcd987b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>