Step over thread exit, always delete the thread non-silently

Step over thread exit, always delete the thread non-silently

With AMD GPU debugging, I noticed that when stepping over a breakpoint
placed on top of the s_endpgm instruction inline (displaced=off), GDB
would behave differently -- it wouldn't print the wave exit.  E.g:

With displaced stepping, or no breakpoint at all:

 stepi
 [AMDGPU Wave 1:4:1:1 (0,0,0)/0 exited]
 Command aborted, thread exited.
 (gdb)

With inline stepping:

 stepi
 Command aborted, thread exited.
 (gdb)

In the cases we see the "exited" notification, handle_thread_exit is
what first called delete_thread on the exiting thread, which is
non-silent.

With inline stepping, however, handle_thread_exit ends up in
update_thread_list (via restart_threads) before any delete_thread
call.  Thus, amd_dbgapi_target::update_thread_list notices that the
wave is gone and deletes it with delete_thread_silent.

This commit fixes it, by making handle_thread_exited call
set_thread_exited (with the default silent=false) early, which emits
the user-visible notification.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I22ab3145e18d07c99dace45576307b9f9d5d966f