gdb: fix {rs6000_nat_target,aix_thread_target}::wait to not use inferior_ptid

gdb: fix {rs6000_nat_target,aix_thread_target}::wait to not use inferior_ptid

Trying to run a simple program (empty main) on AIX, I get:

    (gdb) run
    Starting program: /scratch/simark/build/gdb/a.out
    Child process unexpectedly missing: There are no child processes..
    ../../src/binutils-gdb/gdb/inferior.c:304: internal-error: find_inferior_pid: Assertion `pid != 0' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    ----- Backtrace -----
    0x10ef12a8 gdb_internal_backtrace_1()
            ../../src/binutils-gdb/gdb/bt-utils.c:122
    0x10ef1470 gdb_internal_backtrace()
            ../../src/binutils-gdb/gdb/bt-utils.c:168
    0x1004d368 internal_vproblem(internal_problem*, char const*, int, char const*, char*)
            ../../src/binutils-gdb/gdb/utils.c:396
    0x1004d8a8 internal_verror(char const*, int, char const*, char*)
            ../../src/binutils-gdb/gdb/utils.c:476
    0x1004c424 internal_error(char const*, int, char const*, ...)
            ../../src/binutils-gdb/gdbsupport/errors.cc:55
    0x102ab344 find_inferior_pid(process_stratum_target*, int)
            ../../src/binutils-gdb/gdb/inferior.c:304
    0x102ab4a4 find_inferior_ptid(process_stratum_target*, ptid_t)
            ../../src/binutils-gdb/gdb/inferior.c:318
    0x1061bae8 find_thread_ptid(process_stratum_target*, ptid_t)
            ../../src/binutils-gdb/gdb/thread.c:519
    0x10319e98 handle_inferior_event(execution_control_state*)
            ../../src/binutils-gdb/gdb/infrun.c:5532
    0x10315544 fetch_inferior_event()
            ../../src/binutils-gdb/gdb/infrun.c:4221
    0x10952e34 inferior_event_handler(inferior_event_type)
            ../../src/binutils-gdb/gdb/inf-loop.c:41
    0x1032640c infrun_async_inferior_event_handler(void*)
            ../../src/binutils-gdb/gdb/infrun.c:9548
    0x10673188 check_async_event_handlers()
            ../../src/binutils-gdb/gdb/async-event.c:335
    0x1066fce4 gdb_do_one_event()
            ../../src/binutils-gdb/gdbsupport/event-loop.cc:214
    0x10001a94 start_event_loop()
            ../../src/binutils-gdb/gdb/main.c:411
    0x10001ca0 captured_command_loop()
            ../../src/binutils-gdb/gdb/main.c:471
    0x10003d74 captured_main(void*)
            ../../src/binutils-gdb/gdb/main.c:1329
    0x10003e48 gdb_main(captured_main_args*)
            ../../src/binutils-gdb/gdb/main.c:1344
    0x10000744 main
            ../../src/binutils-gdb/gdb/gdb.c:32
    ---------------------
    ../../src/binutils-gdb/gdb/inferior.c:304: internal-error: find_inferior_pid: Assertion `pid != 0' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n)

This is due to some bit-rot in the AIX port, still relying on the entry
value of inferior_ptid in the wait methods.

Problem #1 is in rs6000_nat_target::wait, here:

      /* Ignore terminated detached child processes.  */
      if (!WIFSTOPPED (status) && pid != inferior_ptid.pid ())
	pid = -1;

At this point, waitpid has returned an "exited" status for some pid, so
pid is non-zero.  Since inferior_ptid is set to null_ptid on entry, the
pid returned by wait is not equal to `inferior_ptid.pid ()`, so we reset
pid to -1 and go to waiting again.  Since there are not more children to
wait for, waitpid then returns -1 so we get here:

      if (pid == -1)
	{
	  gdb_printf (gdb_stderr,
		      _("Child process unexpectedly missing: %s.\n"),
		      safe_strerror (save_errno));

	  /* Claim it exited with unknown signal.  */
	  ourstatus->set_signalled (GDB_SIGNAL_UNKNOWN);
	  return inferior_ptid;
	}

We therefore return a "signalled" status with a null_ptid (again,
inferior_ptid is null_ptid).  This confuses infrun, because if the
target returns a "signalled" status, it should be coupled with a ptid
for an inferior that exists.

So, the first step is to fix the snippets above to not use
inferior_ptid.  In the first snippet, use find_inferior_pid to see if
we know the event process.  If there is no inferior with that pid, we
assume it's a detached child process to we ignore the event.  That
should be enough to fix the problem, because it should make it so we
won't go into the second snippet.  But still, fix the second snippet to
return an "ignore" status.  This is copied from inf_ptrace_target::wait,
which is where rs6000_nat_target::wait appears to be copied from in the
first place.

These changes, are not sufficient, as the aix_thread_target, which sits
on top of rs6000_nat_target, also relies on inferior_ptid.
aix_thread_target::wait, by calling pd_update, assumes that
rs6000_nat_target has set inferior_ptid to the appropriate value (the
ptid of the event thread), but that's not the case.  pd_update
returns inferior_ptid - null_ptid - and therefore
aix_thread_target::wait returns null_ptid too, and we still hit the
assert shown above.

Fix this by changing pd_activate, pd_update, sync_threadlists and
get_signaled_thread to all avoid using inferior_ptid.  Instead, they
accept as a parameter the pid of the process we are working on.

With this patch, I am able to run the program to completion:

    (gdb) r
    Starting program: /scratch/simark/build/gdb/a.out
    [Inferior 1 (process 11010794) exited normally]

As well as break on main:

    (gdb) b main
    Breakpoint 1 at 0x1000036c
    (gdb) r
    Starting program: /scratch/simark/build/gdb/a.out

    Breakpoint 1, 0x1000036c in main ()
    (gdb) c
    Continuing.
    [Inferior 1 (process 26083688) exited normally]

Change-Id: I7c2613bbefe487d75fa1a0c0994423471d961ee9