binfmt_flat: provide an asm-generic/flat.h

This file implements the flat get/put reloc helpers for architectures
that do not need to overload the relocs by simply using get_user/put_user.

Note that many nommu architectures currently use {get,put}_unaligned, which
looks a little bogus and should probably later be switched over to this
version as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>