[POWERPC] Make emergency stack safe for current_thread_info() use

The current_thread_info() macro, used by preempt_count(), assumes the
base address and size of the stack are THREAD_SIZE aligned.

The emergency stack currently isn't either of these things, which
could potentially cause problems anytime we're running on the
emergency stack.  That includes when we detect a bad kernel stack
pointer, and also during early_setup_secondary().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>