rtos: turn stack alignment into a function pointer

rtos: turn stack alignment into a function pointer

Some targets (Cortex M) require more complicated calculations for
turning the stored stack pointer back into a process stack pointer.
For example, the Cortex M stores a bit in the auto-stacked xPSR
indicating that alignment had to be performed and an additional 4
byte padding is present before the exception stacking.  This change
only sets up the framework for Cortex-M unstacking and does not
add Cortex-M support.

Note: this also fixes the alignment calculation nearly addressed by
change #2301 entitled rtos/rtos.c: fix stack alignment calculation.
Updated calculation is in rtos_generic_stack_align.

Change-Id: I0f662cad0df81cbe5866219ad0fef980dcb3e44f
Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Cc: Paul Fertser <fercerpav@gmail.com>
Cc: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Cc: Evan Hunter <evanhunter920@gmail.com>
Cc: Jon Burgess <jburgess777@gmail.com>
Reviewed-on: http://openocd.zylin.com/3002
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins