riscv64: nterp_op_return_void implementation

Test: Run RETURN_VOID against all interpreter tests on a Linux RISC-V VM.

(1) s/-Xint/-Xusejit:false in test/default_run.py:
-    INT_OPTS += " -Xint"
+    INT_OPTS += " -Xusejit:false"

(2) setup
  lunch aosp_riscv64-userdebug

  export ART_TEST_SSH_USER=ubuntu
  export ART_TEST_SSH_HOST=localhost
  export ART_TEST_SSH_PORT=10001
  export ART_TEST_ON_VM=true

  . art/tools/buildbot-utils.sh
  art/tools/buildbot-build.sh --target

  # Create, boot and configure the VM.
  art/tools/buildbot-vm.sh create
  art/tools/buildbot-vm.sh boot
  art/tools/buildbot-vm.sh setup-ssh  # password: 'ubuntu'

  art/tools/buildbot-cleanup-device.sh
  art/tools/buildbot-setup-device.sh
  art/tools/buildbot-sync.sh

(3) test
  art/test.py --target -r --no-prebuild --ndebug --64  -j 8 --cdex-none --interpreter

Change-Id: If53f1f33dc7a9087b62f1ee016c7591459fd4ed2