Improvements to the Cortex-M ports:

- Clear the SysTick current value register before starting the SysTick (only required if something uses SysTick before starting the scheduler).
- Ensure atomic operations are thread safe by executing clrex in the context switch.

FreeRTOS/Source/portable/IAR/ARM_CM3/portasm.s

@@ -91,6 +91,7 @@ xPortPendSVHandler:
 
 	stmdb r0!, {r4-r11}				/* Save the remaining registers. */
 	str r0, [r2]					/* Save the new top of stack into the first member of the TCB. */
+	clrex							/* Ensure thread safety of atomic operations. */
 
 	stmdb sp!, {r3, r14}
 	mov r0, #configMAX_SYSCALL_INTERRUPT_PRIORITY