Introduce windows_nat::event_code_to_string

Instead of:

   switch (event_code)
     {
     case FOO_DEBUG_EVENT:
      DEBUG_EVENTS (..., "FOO_DEBUG_EVENT");
      ...
     case BAR_DEBUG_EVENT:
      DEBUG_EVENTS (..., "BAR_DEBUG_EVENT");
      ...

... with one DEBUG_EVENTS call per event type, log the event just once
before the switch, and introduce a new event_code_to_string function
to handle the event code to string conversion.

Do the same on GDB's and gdbserver's Windows backends.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Id38b7e30df182e4742f3179538de3c643cf42668
commit-id:a8abf6a6