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/CCS/ARM_CM4F/portasm.asm

@@ -123,6 +123,9 @@ xPortPendSVHandler: .asmfunc
 	;/* 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}
 	ldr r0, ulMaxSyscallInterruptPriorityConst
 	ldr r1, [r0]