powerpc/booke: Add Stack Marking support to Booke Exception Prolog

This patch adds a marker to the exception stack frame to aid in debugging.
It's already inserted on other platforms and xmon recognizes it and
identifies exception frames when showing stack traces.

Signed-off-by: Torez Smith <lnxtorez@linux.vnet.ibm.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>