Improvements to the Cortex-M ports:

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.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2465 1d2547de-c912-0410-9cb9-b8ca96c0e9e2