Continue working on the GIC-less Cortex-A5 port for IAR:

Continue working on the GIC-less Cortex-A5 port for IAR:
- Add in the assert when a task attempts to exit its implementing function without deleting itself.
- Remove obsolete code from the context switch asm code (obsoleted by the fact that there is no mask register).
- Attempt to make code more generic by using definitions for additional register addresses.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2283 1d2547de-c912-0410-9cb9-b8ca96c0e9e2