[POWERPC] PPC: Fix xmon stack frame address in backtrace

The stack frame address was being printed incorrectly in the backtrace
option of XMON on PPC.  This patch fixes it to print the actual stack
address instead of the address of the local variable that contains it.

Signed-off-by: Josh Boyer <jdub@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>