riscv64: implement art_jni_dlsym_lookup(_critical)_stub.

Implement art_jni_dlsym_lookup_stub and partly implement
art_jni_dlsym_lookup_critical_stub so that it supports generic JNI (use
T6 register in art_quick_generic_jni_trampoline for the hidden argument
holding the tagged method for @CriticalNative).

Test: run ART test 071-dexfile-map-clean on a Linux RISC-V VM (it failed
  before this patch and it passes with this patch):

  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 --no-image \
    --64 --interpreter 071-dexfile-map-clean

Bug: b/271573990
Co-authored-by: Lifang Xia <lifang_xia@linux.alibaba.com>
Change-Id: I907648bb866e39750c97c5077249a18e3f453e00