Arm call stack structure
This is defined in GDB, GCC and FreeRTOS rather than ARM.  I had some misconceptions about how this worked, and really the stack info I needed was stored in the TCBstub/List pointer in the order specified by the port and the items being pushed onto the stack before context switching.

The order of the 'g' packet is defined by gcc/gdb and can be viewed by the command "maint print registers" from the gdb console.