gdbserver: turn debug_threads into a boolean

debug_threads is always used as a boolean.  Except in ax.cc and
tracepoint.cc.  These files have their own macros that use
debug_threads, and have a concept of verbosity level.  But they both
have a single level, so it's just a boolean in the end.

Remove this concept of level.  If we ever want to re-introduce it, I
think it will be better implemented in a more common location.

Change debug_threads to bool and adjust some users that were treating it
as an int.

Change-Id: I137f596eaf763a08c977dd74417969cedfee9ecf