Added Cortex-M optimised code to the IAR, GCC and Keil Cortex-M port layers.

Added Cortex-M optimised code to the IAR, GCC and Keil Cortex-M port layers.
Tested and updated a few Cortex-M projects to use configUSE_PORT_OPTIMISED_TASK_SELECTION set to 1.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@1782 1d2547de-c912-0410-9cb9-b8ca96c0e9e2