Update Cortex-M3 and Cortex-M4F ports to allow the SysTick to be clocked at a differe...

Update Cortex-M3 and Cortex-M4F ports to allow the SysTick to be clocked at a different speed than the system clock (as is done in the recent STM32L demo. ).
Add additional asserts and isb instructions into the Cortex-M3 and Cortex-M4F ports.