gdb.threads/step-over-thread-exit.exp improvements

gdb.threads/step-over-thread-exit.exp improvements

This commit makes the following improvements to
gdb.threads/step-over-thread-exit.exp:

- Add a third axis to stepping over the breakpoint with displaced vs
  inline stepping -- also test with no breakpoint at all.

- Check that when GDB reports "Command aborted, thread exited.", the
  selected thread is the thread that exited.  This is always true
  currently on GNU/Linux by coincidence, but a similar testcase on AMD
  GPU exposed a problem here.  Better make the testcase catch any
  potential regression.

- Fixes a race that Simon ran into with GDBserver testing.

    (gdb) next
    [New Thread 2143071.2143438]

    Thread 3 "step-over-threa" hit Breakpoint 2, 0x000055555555524e in my_exit_syscall () at .../testsuite/lib/my-syscalls.S:74
    74      SYSCALL (my_exit, __NR_exit)
    (gdb) FAIL: gdb.threads/step-over-thread-exit.exp: displaced-stepping=auto: non-stop=on: target-non-stop=on: schedlock=off: cmd=next: ns_stop_all=0: command aborts when thread exits

  I was not able to reproduce it, but I believe that what happens is
  the following:

  Once we continue, the thread 2 exits, and the main thread thus
  unblocks from its pthread_join, and spawns a new thread.  That new
  thread may hit the breakpoint at my_exit_syscall very quickly.  GDB
  could then see/process that breakpoint event before the thread exit
  event for the thread we care about, which would result in the
  failure seen above.

  The fix here is to not loop and start a new thread at all in the
  scenario where the race can happen.  We only need to loop and spawn
  new threads when testing with "cmd=continue" and schedlock off, in
  which case GDB doesn't abort the command when the thread exits.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I90c95c32f00630a3f682b1541c23aff52451f9b6