[MIPS] Fix KMODE for the R3000

 This must be the oldest bug that we have got.  Leaving interrupts "as
they are" for the R3000 obviously means copying IEp to IEc.  Since we have
got STATMASK now, I took this opportunity to mask the status register
"correctly" for the R3000 now too.  Oh, and the R3000 hardly ever is
64-bit.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>