arch/tile: avoid accidentally unmasking NMI-type interrupt accidentally

The return path as we reload registers and core state requires that r30
hold a boolean indicating whether we are returning from an NMI, but in a
couple of cases we weren't setting this properly, with the result that we
could accidentally unmask the NMI interrupt(s), which could cause confusion.
Now we set r30 in every place where we jump into the interrupt return path.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>