x86: Start using native object relocatable files in parasite code

Since at the moment we're running only x86-64 not 32 bit tasks,
and our code is not carrying any big statically defined structures
we can use relocatable files directly with all relocation applied
during building.

This gonna be changed soon once we start supporting 32 bit tasks.

IOW even currently we need (which is not yet done but it's safe)

 - check for gotpcrel relocations
 - apply relocations with generated elf_apply_relocs helper

Currently overall scheme looks this way

 - our object files are linked together into parasite.built-in.bin.o file
 - then pie/piegen/piegen tool is called which parses this file and generates
   C source code file with bytestream and all information needed to rellocate
   this bytestream into a new place (see elf_apply_relocs helper)

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