[POWERPC] spufs: implement error event delivery to user space

This tries to fix spufs so we have an interface closer to what is
specified in the man page for events returned in the third argument of
spu_run.

Fortunately, libspe has never been using the returned contents of that
register, as they were the same as the return code of spu_run (duh!).

Unlike the specification that we never implemented correctly, we now
require a SPU_CREATE_EVENTS_ENABLED flag passed to spu_create, in
order to get the new behavior. When this flag is not passed, spu_run
will simply ignore the third argument now.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>