rtos: add instructions and helper code to make FreeRTOS work again

rtos: add instructions and helper code to make FreeRTOS work again

Run-time tested with FreeRTOS V8.1.2 (current version).

For the time being I propose this way of dealing with RTOSes that do
not export necessary information on their own.

I also suggest implementing a similar scheme for ChibiOS, exporting
the necessary struct fields' offsets via an OpenOCD-specific helper.

Change-Id: Iacf8b88004d62206215fe80011fd7592438446a3
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2347
Tested-by: jenkins