Ensure selected thread after thread exit stop

While making step over thread exit work properly on AMDGPU, I noticed
that if there's a breakpoint on top of the exit syscall, and,
displaced stepping is off, then when GDB reports "Command aborted,
thread exited.", GDB also switches focus to a random thread, instead
of leaving the exited thread as selected:

 (gdb) thread
 [Current thread is 6, lane 0 (AMDGPU Lane 1:4:1:1/0 (0,0,0)[0,0,0])]
 (gdb) si
 Command aborted, thread exited.
 (gdb) thread
 [Current thread is 5 (Thread 0x7ffff626f640 (LWP 3248392))]
 (gdb)

The previous patch extended gdb.threads/step-over-thread-exit.exp to
exercise this on GNU/Linux (on the CPU side), and there, after that
"si", we always end up with the exiting thread as selected even
without this fix, but that's just a concidence, there's a code path
that happens to select the exiting thread for an unrelated reason.

This commit add the explict switch, fixing the latent problem for
GNU/Linux, and the actual problem on AMDGPU.  I wrote a gdb.rocm/
testcase for this, but it can't be upstreamed yet, until more pieces
of the DWARF machinery are upstream as well.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I6ff57a79514ac0142bba35c749fe83d53d9e4e51

gdb/infrun.c

@@ -5895,7 +5895,14 @@ handle_thread_exited (execution_control_state *ecs)
 
   if (abort_cmd)
     {
+      /* We're stopping for the thread exit event.  Switch to the
+	 event thread again, as finish_step_over may have switched
+	 threads.  */
+      switch_to_thread (ecs->event_thread);
+
+      /* Emit [Thread ... exited] notification.  */
       delete_thread (ecs->event_thread);
+
       ecs->event_thread = nullptr;
       return false;
     }