nios2: implement fiberRtnValueSet()

The return value of _Swap() is often treated as a "don't care" value and thus
often ignored. However, there are cases when it is desirable to have a
meaningful return value. This meaningful value can be assigned via
fiberRtnValueSet(). To that end, a new field has been added to the coop
register struct to store this value for when _Swap() needs to return that
meaningful value.

Change-Id: Ic4967fa7d602850c09ebde18e8bfd4c97cda9ec8
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>