binfmt_flat: remove the uapi <linux/flat.h> header

The split between the two flat.h files is completely arbitrary, and the
uapi version even contains CONFIG_ ifdefs that can't work in userspace.
The only userspace program known to use the header is elf2flt, and it
ships with its own version of the combined header.

Use the chance to move the <asm/flat.h> inclusion out of this file, as it
is in no way needed for the format defintion, but just for the binfmt
implementation.

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