- The Alpha fast-soft-intrs implementation supports 2 soft interrupt levels,
  so in exception_return() ensure we clear ineligible-at-new-IPL softint
  bits from the SSIR, otherwise we could loop forever in the following
  scenario:

processing softnet -> clock interrupt -> schedule softclock

- If the softint thread blocks, it's possible to bounce back through
  the softint return trampoline at something other than IPL_HIGH.  This
  is not a problem other than it's contrary to what alpha_softint_dispatch()
  expect, so make alpha_softint_return() go to IPL_HIGH before it does
  anything else.

These two fixes make fast-soft-interrupts work on Alpha.

(thorpej)