um: set __HAVE_ARCH_GATE_AREA for x86_64

Implement arch_vma_name() and make get_gate_vma(), in_gate_area() and
in_gate_area_no_mm() a nop.

We need arch_vma_name() to support vDSO.

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>