pie/vdso: convert (char *mem) to (uintptr_t mem)

Impact: drop additional casts all around. Cleanup.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>