gdbserver/Windows: Fix "no program to debug" error

Trying to start a program with GDBserver on Windows yields
the following error:

    $ gdbserver.exe --once :4444 simple_main.exe
    Killing process(es): 5008
    No program to debug
    Exiting

The error itself comes from the following code shortly after
create_inferior gets called (in server.c::main):

    /* Wait till we are at first instruction in program.  */
    create_inferior (program_path.get (), program_args);
    [...]

    if (last_status.kind == TARGET_WAITKIND_EXITED
        || last_status.kind == TARGET_WAITKIND_SIGNALLED)
      was_running = 0;
    else
      was_running = 1;

    if (!was_running && !multi_mode)
      error ("No program to debug");

What happens is that the "last_status" global starts initialized
as zeroes, which means last_status.kind == TARGET_WAITKIND_EXITED,
and we expect create_inferior to be waiting for the inferior to
start until reaching the SIGTRAP, and to set the "last_status"
global to match that last event we received.

I suspect this is an unintended side-effect of the following change...

    commit 2090129c
    Date:   Thu Dec 22 21:11:11 2016 -0500
    Subject: Share fork_inferior et al with gdbserver

... which removes some code in server.c that was responsible for
starting the inferior in a functin that was named start_inferior,
and looked like this:

   signal_pid = create_inferior (new_argv[0], &new_argv[0]);
   [...]
   /* Wait till we are at 1st instruction in program, return new pid
      (assuming success).  */
   last_ptid = mywait (pid_to_ptid (signal_pid), &last_status, 0, 0);

The code has been transitioned to using fork_inferior, but sadly,
only for the targets that support it. On Windows, the calls to wait
setting "last_status" simply disappeared.

This patch adds it back in the Windows-specific implementation of
create_inferior.

gdb/gdbserver/ChangeLog:

        PR server/23158:
        * win32-low.c (win32_create_inferior): Add call to my_wait
        setting last_status global.