nios2: implement _Swap() and _new_thread()

With this code we can successfully boot and context switch into
the main thread. Nanokernel hello_world has the expected
"Hello World!" string in the RAM console.

Change-Id: I56335d992f5a7cbb12d9e4c02d1cc23ea28ae6ef
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>