um: disable scan_elf_aux() on x86_64

Reusing the host's vDSO makes only sense on x86_32.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>