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_CM4F/portasm.s

@@ -101,6 +101,9 @@ xPortPendSVHandler:
 	/* Save the new top of stack into the first member of the TCB. */
 	str r0, [r2]
 
+	/* Ensure thread safety of atomic operations. */
+	clrex
+
 	stmdb sp!, {r3}
 	mov r0, #configMAX_SYSCALL_INTERRUPT_PRIORITY
 	msr basepri, r0