rtos: add support for uC/OS-III

This patch introduces RTOS support for uC/OS-III. Currently, only
FPU-less ARM Cortex-M targets are supported. Due to the configurability
of the RTOS, an OpenOCD-specific file must be linked along with the
project to determine the correct offsets within the OS_TCB structure.

In addition to the above, a crash was fixed in rtos_get_gdb_reg_list
such that RTOS support could be used between resets without restarting
OpenOCD and support for the Hg packet was cleaned up.

Change-Id: Ide004a689e6b886185df665c00fb644629eb31d1
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/3556


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>