powerpc: Fix graceful debugger recovery

When exiting xmon with 'x' (exit and recover), oops_begin bails
out immediately, but die then calls __die() and oops_end(), which
cause a lot of bad things to happen.

If the debugger was attached then went to graceful recovery, exit
from die() immediately.

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