[POWERPC] Stacktrace support for lockdep

This adds stacktrace support for powerpc, which will be needed for
lockdep.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>