x86-64 has special case code for in_gate_area(), but it is not clean.

* Replace CONFIG_ARCH_GATE_AREA with __HAVE_ARCH_GATE_AREA.
  ARCH_GATE_AREA is not a config option.

* The definitions of get_gate_vma() and in_gate_area() are identical in
  include/asm-x86_64/page.h and include/linux/mm.h.  Fold the duplicate
  definitions into include/linux/mm.h.

Does not affect kallsyms directly, this patch just creates a clean base
for patch 2.
Signed-off-by: Keith Owens <kaos@ocs.com.au>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>