[POWERPC] Check that the syscall table matches the syscall numbers

Also check that __NR_syscalls has been updated appropriately.

Hopefully this will catch any out of order additions to the
table in the future.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>