Revert some of "binfmt_elf: cleanups"

The commit "binfmt_elf: cleanups"
(f670d0ecda73b7438eec9ed108680bc5f5362ad8) removed an ifndef elf_map but
this breaks compilation for metag which does define elf_map.

This adds the ifndef back in as it was before, but does not affect the
other cleanups made by that patch.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Acked-by: Mikael Pettersson <mikpe@it.uu.se>