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>

src/target/target.c

@@ -228,6 +228,7 @@ static const Jim_Nvp nvp_target_debug_reason[] = {
 	{ .name = "watchpoint-and-breakpoint", .value = DBG_REASON_WPTANDBKPT },
 	{ .name = "single-step"              , .value = DBG_REASON_SINGLESTEP },
 	{ .name = "target-not-halted"        , .value = DBG_REASON_NOTHALTED  },
+	{ .name = "program-exit"             , .value = DBG_REASON_EXIT },
 	{ .name = "undefined"                , .value = DBG_REASON_UNDEFINED },
 	{ .name = NULL, .value = -1 },
 };