In the end, it wasn't a cache coherency problem, nor a DMA corruption problem,
nor even a SONIC driver problem.  It was locore_mips3.S:mips3_UserIntr().
You see, this function makes use of MIPS_HARD_INT_MASK, which I neglected
to mask out INT_MASK_4 in cpuregs.h!!!  I was only doing it for MIPS_INT_MASK,
but there was this one place where MIPS_HARD_INT_MASK was being used and
unfielded INT4's were slipping by, randomly hanging processes in user-space.

So now that I can successfully boot to a shell on my Magnum, I'm going to
see if I can set up NetBSD on an internal scsi disk.  And, of course, I'll
post my patches to 1.5R shortly.  It's quite satisfying to know that the
port-arc webpage is actually telling the truth now, when it claims that
the Magnum is "supported".  :)

Cheers,
-Mark