gdb/remote: handle attach when stop packet lacks thread-id

Bug PR gdb/28405 reports a regression when using attach with an
extended-remote target.  In this case the target is not including a
thread-id in the stop packet it sends back after the attach.

The regression was introduced with this commit:

  commit 8f66807b98f7634c43149ea62e454ea8f877691d
  Date:   Wed Jan 13 20:26:58 2021 -0500

      gdb: better handling of 'S' packets

The problem is that when GDB processes the stop packet, it sees that
there is no thread-id and so has to "guess" which thread the stop
should apply to.

In this case the target only has one thread, so really, there's no
guessing needed, but GDB still runs through the same process, this
shouldn't cause us any problems.

However, after the above commit, GDB now expects itself to be more
internally consistent, specifically, only a thread that GDB thinks is
resumed, can be a candidate for having stopped.

It turns out that, when GDB attaches to a process through an
extended-remote target, the threads of the process being attached too,
are not, initially, marked as resumed.

And so, when GDB tries to figure out which thread the stop might apply
too, it finds no threads in the processes marked resumed, and so an
assert triggers.

In extended_remote_target::attach we create a new thread with a call
to add_thread_silent, rather than remote_target::remote_add_thread,
the reason is that calling the latter will result in a call to
'add_thread' rather than 'add_thread_silent'.  However,
remote_target::remote_add_thread includes additional
actions (i.e. calling remote_thread_info::set_resumed and set_running)
which are missing from extended_remote_target::attach.  These missing
calls are what would serve to mark the new thread as resumed.

In this commit I propose that we add an extra parameter to
remote_target::remote_add_thread.  This new parameter will force the
new thread to be added with a call to add_thread_silent.  We can now
call remote_add_thread from the ::attach method, the extra
actions (listed above) will now be performed, and the thread will be
left in the correct state.

Additionally, in PR gdb/28405, a segfault is reported.  This segfault
triggers when 'set debug remote 1' is used before trying to reproduce
the original assertion failure.  The cause of this is in
remote_target::select_thread_for_ambiguous_stop_reply, where we do
this:

  remote_debug_printf ("first resumed thread is %s",
		       pid_to_str (first_resumed_thread->ptid).c_str ());
  remote_debug_printf ("is this guess ambiguous? = %d", ambiguous);

  gdb_assert (first_resumed_thread != nullptr);

Notice that when debug printing is on we dereference
first_resumed_thread before we assert that the pointer is not
nullptr.  This is the cause of the segfault, and is resolved by moving
the assert before the debug printing code.

I've extended an existing test, ext-attach.exp, so that the original
test is run multiple times; we run in the original mode, as normal,
but also, we now run with different packets disabled in gdbserver.  In
particular, disabling Tthread would trigger the assertion as it was
reported in the original bug.  I also run the test in all-stop and
non-stop modes now for extra coverage, we also run the tests with
target-async enabled, and disabled.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28405

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