Add ehb instructions back into PIC32 port layer (upon advice).

Add ehb instructions back into PIC32 port layer (upon advice).
Add configCLEAR_TICK_TIMER_INTERRUPT into PIC32 port layer to allow the timer configuration to be changed without any edits to the port layer being required.
Add prvTaskExitError() into the PIC32 port layer to trap tasks that attempt to exit from their implementing function.
Provide the ability to trap interrupt stack overflows in the PIC32 port.
Radically improve the timing in the Win32 simulator port layer.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2087 1d2547de-c912-0410-9cb9-b8ca96c0e9e2