gdb: fix extended-remote restart

gdb: fix extended-remote restart

Seems versions of gdb > 6.8 require an W stop reply after receiving a
kill packet.

Without this we receive the following error from gdb:
gdb/thread.c:72: internal-error: inferior_thread: Assertion `tp' failed.

Change-Id: I86765a321f0429c9b517fe13ded0ee2dbd4b2f87
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/911
Tested-by: jenkins
Reviewed-by: Joel Bodenmann <joel@unormal.org>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>