Windows gdb: Watchpoints while running (internal vs external stops)

Windows gdb: Watchpoints while running (internal vs external stops)

Teach the Windows target to temporarily pause all threads when we
change the debug registers for a watchpoint.  Implements the same
logic as Linux uses:

    ~~~
      /*                             (...) if threads are running when the
         mirror changes, a temporary and transparent stop on all threads
         is forced so they can get their copy of the debug registers
         updated on re-resume.   (...)  */
    ~~~

On Linux, we send each thread a SIGSTOP to step them.  On Windows,
SuspendThread itself doesn't cause any asynchronous debug event to be
reported.  However, we've implemented windows_nat_target::stop such
that it uses SuspendThread, and then queues a pending GDB_SIGNAL_0
stop on the thread.  That results in a user-visible stop, while here
we want a non-user-visible stop.  So what we do is re-use that
windows_nat_target::stop stopping mechanism, but add an external vs
internal stopping kind distinction.  An internal stop results in
windows_nat_target::wait immediately re-resuming the thread.

Note we don't make the debug registers poking code SuspendThread ->
write debug registers -> ContinueThread itself, because SuspendThread
is actually asynchronous and may take a bit to stop the thread (a
following GetThreadContext blocks until the thread is actually
suspended), and, there will be several debug register writes when a
watchpoint is set, because we have to set all of DR0, DR1, DR2, DR3,
and DR7.  Defering the actualy writes to ::wait avoids a bunch of
SuspendThread/ResumeThread sequences, so in principle should be
faster.

Change-Id: I39c2492c7aac06d23ef8f287f4afe3747b7bc53f