Updates to the Cortex-M tickless idle code to reduce clock slippage.

Updates to the Cortex-M tickless idle code to reduce clock slippage.
Updates to prevent the vTaskSwitchContext() function being removed from GCC builds when link time optimisation is used.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2493 1d2547de-c912-0410-9cb9-b8ca96c0e9e2