Build sdk variants of libc++ for riscv64 am: 69c72cf1ae

Original change: https://android-review.googlesource.com/c/platform/external/libcxx/+/2404996

Change-Id: I051e1c644b0aaa55f33520271078e374ba820c3f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>