powerpc/64: hard disable irqs on the panic()ing CPU

Similar to previous patches, hard disable interrupts when a CPU is
in panic. This reduces the chance the watchdog has to interfere with
the panic, and avoids any other type of masked interrupt being
executed when crashing which minimises the length of the crash path.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>