vdso: Make parasite_fixup_vdso being arch implemented

parasite_fixup_vdso is not worth calling on arm at all,
moreover it might try to read pfn for addresses greater
than TASK_SIZE, leading to dump interruption. Lets make
it arch dependent.

[alekskartashov@: Fix compiling on arm]

Reported-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Tested-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>