[MIPS] R3000 setup for kernel_thread()

 Match the R4000 semantics for the initial state of interrupt/kernel
status register flags for the R3000 in kernel_thread().

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