Retrieves the priority value for this thread. The thread priority value is one of the following:
%THREAD_PRIORITY_IDLE = -15
%THREAD_PRIORITY_LOWEST = -2
%THREAD_PRIORITY_BELOW_NORMAL = -1
%THREAD_PRIORITY_NORMAL = 0
%THREAD_PRIORITY_ABOVE_NORMAL = +1
%THREAD_PRIORITY_HIGHEST = +2
%THREAD_PRIORITY_TIME_CRITICAL= +15