pie: piegen -- Slightly rework the building procedure

 - Move relocs application into a separate file which get
   compiled as a regular C file in criu (pie/pie-relocs.[ch])
 - Move types used by piegen into pie/piegen/uapi/types.h

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