[POWERPC] Separate out legacy machine check exception parsers

Move out the old-style exception parsers to a separate function, and
don't call it on platforms that have a platform-specific handler.

It would make sense to move out the generic versions into their platforms
instead, but that can be done gradually down the road.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>