bsps: BSP_INTERRUPT_STACK_AT_WORK_AREA_BEGIN

Remove the BSP_INTERRUPT_STACK_AT_WORK_AREA_BEGIN hack.  The interrupt
stacks are now allocated by the linker.

Update #3459.

bsps/powerpc/mpc55xxevb/start/bspstart.c

@@ -90,7 +90,7 @@ void bsp_start(void)
 
 	/* Initialize exceptions */
 	ppc_exc_initialize_with_vector_base(
-		(uintptr_t) bsp_section_work_begin,
+		(uintptr_t) _Configuration_Interrupt_stack_area_begin,
 		rtems_configuration_get_interrupt_stack_size(),
 		mpc55xx_exc_vector_base
 	);