[POWERPC] Unify ppc syscall tables

Avoid duplication of the syscall table for the cell platform.  Based on an
idea from David Woodhouse.

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