Add assembler for riscv64, part 3. am: e7dd044ecf am: 6d4c064043

Original change: https://android-review.googlesource.com/c/platform/art/+/2596790

Change-Id: I52761d9f3d4a42e1cbb095e2b152020ae7289478
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>