gdbserver/win32-low.cc: remove use of `all_threads`

gdbserver/win32-low.cc: remove use of `all_threads`

Fix this:

    gdbserver/win32-low.cc: In function ‘void child_delete_thread(DWORD, DWORD)’:
    gdbserver/win32-low.cc:192:7: error: ‘all_threads’ was not declared in this scope; did you mean ‘using_threads’?
      192 |   if (all_threads.size () == 1)
          |       ^~~~~~~~~~~
          |       using_threads

Commit 9f77b3aa0bfc ("gdbserver: change 'all_processes' and
'all_threads' list type") changed the type of `all_thread` to an
intrusive_list, without changing this particular use, which broke the
build because an intrusive_list doesn't know its size, so it doesn't
have a `size()` method.  The subsequent commit removed `all_threads`,
leading to the error above.

Fix it by using the number of threads of the concerned process instead.
My rationale: as far as I know, GDBserver on Windows only supports one
process at a time, so there's no need to iterate over all processes.  If
we made GDBserver for Windows support multiple processes, my intuition
is that we'd want this check to use the number of threads of the
concerned process, not the number of threads overall.

Add the method `process_info::thread_count`, to get the number of
threads of the process.

I'm not really sure what this check is for in the first place, Hannes
Domani said that this check didn't seem to trigger on Windows 7 and 11.
Perhaps it was necessary before.

Change-Id: I84d6226532b887d99248cf3be90f5065fb7a074a
Tested-By: Hannes Domani <ssbssa@yahoo.de>