gdb_server: Include current RTOS thread in signal packets

This allows GDB to automatically switch to the thread that has
been interrupted and show you where it has stopped.

Change-Id: Icb9500dc42a61eb977e9fac55ce9503c9926bf5d
Signed-off-by: Jon Burgess <jburgess777@gmail.com>
Reviewed-on: http://openocd.zylin.com/2303
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>