As compel is an external tool now, let's treat it as such -- remove the
hardcoded paths to compel files, they are now provided via compel CLI.

Details:

1. The compel std plugin dependency is dropped from the intermediate
criu/Makefile. It was there in order to make sure plugin is built before
pie, but the top level Makefile and Makefile.compel take care about it.

2. The compel std plugin logic is simplified in criu/pie/Makefile.

3. The compel linker script dependency is kept as-is for now (except
the linker script file name no longer appears in the list of objects
to link).

4. Linking of compel std plugin is moved to a second linking stage
in order to simplify the Makefile.

Side note: we can actually avoid running the linker twice!

5. The compel binary dependency is dropped from criu/pie/Makefile
as this is already there in top-level Makefile (look for criu-deps).

[v2: reworked after dsafonov@ comments; keep most compel deps]
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>