gdb server: new feature, add stop reason in stop reply packet for gdb

gdb server: new feature, add stop reason in stop reply packet for gdb

In GDB remote serial protocol, the stop reply packet could contain more
detail stop reason. The currently defined stop reasons are listed below.

* watch
* rwatch
* awatch
* library
* replaylog

This commit adds stop reason, watch/rwatch/awatch, in stop reply packet for
just hit watchpoint. As manual indicates, at most one stop reason should be present.

The function needs target to implement new hook, hit_watchpoint. The hook will fill
the hit watchpoint in second parameter. The information will assist gdb to locate
the watchpoint. If no such information, gdb needs to scan all watchpoints by itself.

Refer to GDB Manual, D.3 Stop Reply Packets

Change-Id: I1f70a1a9cc772e88e641b6171f1a009629a43bd1
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1092
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>