Conditional compilation of the ADA (gnat) self TCB variable.

[Chris Johns]

To quote Joel:

Add --enable-ada and make the gnat self variable follow
that setting just like some of the C++ libraries do.
If you don't use Ada, there is no point in paying the price.

This would conditionalize a variable, a TCB field, and a
couple of lines in _Thread_Dispatch.

Release:
cvs-head

[Joel Sherrill]

State-Changed-From-To: open->closed
State-Changed-Why: Adding this turned out to be controversial due to the addition

build options. We can revisit it later when RTEMS gets a more
fine grained configuration scheme. :)