Define NR_getrandom for riscv64 am: 317175f862

Original change: https://android-review.googlesource.com/c/platform/external/boringssl/+/2461373

Change-Id: Iee4a1e70c468e637a4e753365786b42b4fb5faa8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>