vdso: Initialize vdso data on startup

During criu startup we need to fill symbol table of own
run-time vdso provided by the kernel. We will need this
data for vdso proxy.

Because this functions are not used in restorer code,
we move them out of PIE (since PIE code must remain
as small as possible).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>