unified/x86: add unified kernel support for x86 arch

The x86 architecture port is fitted with support for the unified kernel,
namely:

- the interrupt exit code now calls _Swap() if the current
  thread is not a coop thread and if the scheduler is not locked

- there is no 'task' fields in the _nanokernel anymore: _Swap()
  now calls _get_next_ready_thread instead

- the _nanokernel.fiber field is replaced by a more sophisticated
  ready_q, based on the microkernel's priority-bitmap-based one

- nano_private includes nano_internal.h from the unified directory

- the FIBER, TASK and PREEMPTIBLE flags do not exist anymore: the thread
  priority drives the behaviour

- the tcs uses a dlist for queuing in both ready and wait queues instead
  of a custom singly-linked list

- other new fields in the tcs include a schedule-lock count, a
  back-pointer to init data (when the task is static) and a pointer to
  swap data, needed when a thread pending on _Swap() must be passed more
  then just one value (e.g. k_stack_pop() needs an error code and data)

- fiberRtnValueSet() is aliased to _set_thread_return_value since it
  also operates on preempt threads now

- _set_thread_return_value_with_data() sets the swap_data field in
  addition to a return value from _Swap()

- convenience aliases are created for shorter names:

  - _current is defined as _nanokernel.current
  - _ready_q is defined as _nanokernel.ready_q

- _Swap() sets the threads's return code to -EAGAIN before swapping out
  to prevent timeouts to have to set it (solves hard issues in some
  kernel objects).

- Floating point support.

Note that, in _Swap(), the register holding the thread to be swapped in has
been changed from %ecx to %eax in both the legacy kernel and the unified kernel
to take advantage of the fact that the return value of _get_next_ready_thread()
is stored in %eax, and this avoids moving it to %ecx.

Work by: Dmitriy Korovkin <dmitriy.korovkin@windriver.com>
         Allan Stephens <allan.stephens@windriver.com>
	 Benjamin Walsh <benjamin.walsh@windriver.com>

Change-Id: I4ce2bd47bcdc62034c669b5e889fc0f29480c43b
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>