riscv64: Implement `NullPointerHandler`.

Test: # Edit `run-test` to disable checker, then
      testrunner.py --target --64 --ndebug --optimizing
      # Ignore 7 pre-existing failures.
Bug: 283082089
Change-Id: If7ca0546a4f2a3576002bc2586b1a8d4892bb6c2
5 files changed