Replace the static prvSetupTimerInterrupt() function in the Cortex-M port layers...

Replace the static prvSetupTimerInterrupt() function in the Cortex-M port layers that still used it (other than MPU ports so far) with a weakly defined function call vPortSetupTimerInterrupt() - which allows application writers to override the function with one that uses a different clock.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2779 1d2547de-c912-0410-9cb9-b8ca96c0e9e2