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

src/rtos/rtos.h

@@ -83,7 +83,15 @@ struct rtos_register_stacking {
 	unsigned char stack_registers_size;
 	signed char stack_growth_direction;
 	unsigned char num_output_registers;
-	unsigned char stack_alignment;
+	/* Some targets require evaluating the stack to determine the
+	 * actual stack pointer for a process.  If this field is NULL,
+	 * just use stacking->stack_registers_size * stack_growth_direction
+	 * to calculate adjustment.
+	 */
+	int64_t (*calculate_process_stack)(struct target *target,
+		const uint8_t *stack_data,
+		const struct rtos_register_stacking *stacking,
+		int64_t stack_ptr);
 	const struct stack_register_offset *register_offsets;
 };