allows all the INTR_FILTER #ifdef's to be removed from the MD interrupt
code.
- Rename the intr_event 'eoi', 'disable', and 'enable' hooks to
  'post_filter', 'pre_ithread', and 'post_ithread' to be less x86-centric.
  Also, add a comment describe what the MI code expects them to do.
- On amd64, i386, and powerpc this is effectively a NOP.
- On arm, don't bother masking the interrupt unless the ithread is
  scheduled in the non-INTR_FILTER case to match what INTR_FILTER did.
  Also, don't bother unmasking the interrupt in the post_filter case if
  we never masked it.  The INTR_FILTER case had been doing this by having
  arm_unmask_irq for the post_filter (formerly 'eoi') hook.
- On ia64, stray interrupts are now masked for the non-INTR_FILTER case.
  They were already masked in the INTR_FILTER case.
- On sparc64, use the a NULL pre_ithread hook and use intr_enable_eoi() for
  both the 'post_filter' and 'post_ithread' hooks to match what the
  non-INTR_FILTER code did.
- On sun4v, retire the ithread wrapper hack by using an appropriate
  'post_ithread' hook instead (it's what 'post_ithread'/'enable' was
  designed to do even in 5.x).

Glanced at by:	piso
Reviewed by:	marius
Requested by:	marius [1], [5]
Tested on:	amd64, i386, arm, sparc64