riscv64: implement tests 004-SignalTest and 115-native-bridge.

Also, make the tests fail on unsupported architectures (instead of being
ignored and marked as passed).

Bug: 271573990
Test: run ART test 004-SignalTest and 115-native-bridge on a Linux
  RISC-V VM (note that 115-native-bridge is skipped on target):

  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

  art/test.py --target -r --no-prebuild --ndebug --64 --interpreter \
    004-SignalTest 115-native-bridge

Change-Id: I31c22f0acf2ecc84752f315b304fb6a51c1b3bea
2 files changed