all-stop/synchronous RSP support thread-exit events

all-stop/synchronous RSP support thread-exit events

Currently, GDB does not understand the THREAD_EXITED stop reply in
remote all-stop mode.  There's no good reason for this, it just
happened that THREAD_EXITED was only ever reported in non-stop mode so
far.  This patch teaches GDB to parse that event in all-stop RSP too.
There is no need to add a qSupported feature for this, because the
server won't send a THREAD_EXITED event unless GDB explicitly asks for
it, with QThreadEvents, or with the GDB_TO_EXIT QThreadOptions option
added in the next patch.

Change-Id: Ide5d12391adf432779fe4c79526801c4a5630966