vdso: aarch64 -- Merge fixes from x86

This brings the changes made in the following commits to the
aarch64 copy of the code.

commit 7794f67f
Author: Cyrill Gorcunov <gorcunov@openvz.org>
Date:   Tue Aug 5 13:59:18 2014 +0400

    vdso: x86 -- Fix missing ability to remap vDSO if only one zone present

commit 066add0d
Author: Cyrill Gorcunov <gorcunov@openvz.org>
Date:   Tue Aug 5 13:07:00 2014 +0400

    vdso: x86 -- Simplify vdso_proxify

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>