powerpc: Fix various compile errors with ARCH=ppc, ppc64 and powerpc

This makes ppc use the syscalls.c from arch/powerpc/kernel, exports
copy_and_flush from head_32.S for use by prom_init.c (ARCH=powerpc),
and consolidates the sys_fadvise64_64 implementations for 32-bit.

Signed-off-by: Paul Mackerras <paulus@samba.org>