SMP Documentation (#280)

SMP Documentation (#280)

* Add XMOS XCore ports

Some minor modifications are also made to the kernel to support the
XCore compiler's automatic stack size calculation.

* Update kernel to support SMP

The XMOS XCore ports are also updated to support SMP.

* Fix compiler warnings in xcore ports

The port set and clear interrupt mask from ISR macros were removed from
the ports so that the default macros found in FreeRTOS.h are used
instead. The default macros do not result in warnings when they are
used.

* Remove inline function from timers.h

Inline function converted to macro. This should now build when
optimizations are off and inlining is disabled.

* Fix compiler warnings in xcore ports and tasks.c

* fixed documentation for ulTaskNotifyTake() and ulTaskNotifyTakeIndexed()

* spelling fixes for tasks.c

* adding the kernel SMP documentation

Co-authored-by: Michael Bruno <mikeb@xmos.com>