Thread-local storage should be usable once an idle thread exists

[Sebastian Huber]

At some point during system initialization the idle threads are created. Afterwards the boot processor basically executes within the context of an idle thread with thread dispatching disabled. On some architectures, the thread-local storage state must be applied to processor registers. Add a new CPU port function to do this: _CPU_Use_thread_local_storage( Context_Control *executing ).

[Sebastian Huber <sebastian.huber@…>]

In d07d6498/rtems:

samples/cdtest: Test exceptions during system init

Update #4668.
Update #4672.

[Sebastian Huber <sebastian.huber@…>]

In 03e4d1e9/rtems:

score: Add _CPU_Use_thread_local_storage()

At some point during system initialization, the idle threads are created.
Afterwards, the boot processor basically executes within the context of an idle
thread with thread dispatching disabled. On some architectures, the
thread-local storage area of the associated thread must be set in dedicated
processor registers. Add the new CPU port function to do this:

void _CPU_Use_thread_local_storage( const Context_Control *context )

Close #4672.