gdb_server: add target_debug_reason for program exit detection

gdb_server: add target_debug_reason for program exit detection

Currently, there is no way to notify gdb that program has exited.
Add new target_debug_reason called DBG_REASON_EXIT to notify gdb
the condition has occured. If the debug reason is DBG_REASON_EXIT,
gdb_server will send 'W' packet to tell gdb the process has exited.

Change-Id: I7a371da292716a3e6ac4cc2c31b009a651fe047a
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1242
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>