riscv64: define instruction set features. am: 910c95fe2d

Original change: https://android-review.googlesource.com/c/platform/art/+/2421275

Change-Id: I723f1276d08df2e518aa640adb7ab66b13fb2967
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>