should be possible to clear resumeack

[timsifive]

From today's meeting:

• Scenario:
  • Halted
  • Hart resumes due to external trigger
  • Hart hits breakpoint
  • Debugger doesn’t know the hart resumed (so register caches need to be flushed)
• Possible solution
  • Optional, backwards compatible, imperfect
    • Resumeack is automatically set when resuming for any reason
    • Add new bit in dmcs2 that lets debugger clear resumeack while halted
  • Halt acknowledge handshake, backwards incompatible, handles all races
    • For v1.1