compel: make plugins .a archives

The objective is to only do parasite code linking once -- when we link
parasite objects with compel plugin(s). So, let's use ar (rather than
ld) here. This way we'll have a single ld invocation with the proper
flags (from compel ldflags) etc.

There are two tricks in doing it:

1. The order of objects while linking is important. Therefore, compel
   plugins should be the last to add to ld command line.

2. Somehow ld doesn't want to include parasite-head.o in the output
   (probably because no one else references it), so we have to force
   it in with the modification to our linker scripts.

NB: compel makefiles are still a big mess, but I'll get there.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>