criu-coredump: merge criu-coredump project

This patch introduces a new subproject called "criu-coredump". The name
is pretty much self-explanatory, it allows generating coredumps from
criu images. Coredumps are readable and usable by gdb. The structure is
something in the middle of kernel-generated and gdb-generated core dumps.

This is a first iteration of core-dump patch series. It introduces
ability to generate gdb-operatable core dump only for x86_64 machines.
To support other architectures there will be a set to introduce proper
structures for notes section of the core dump. It is also worth noting,
that we still leave some notes(SIGINFO primarily) not fullfilled, as
I still need to figure out what to put in there in our case, where no
signal is involved in triggering core dump generation and other
caveats like zeroed vvar(we have problems with it withing CRIU itself
as well as gdb has some difficalties with it) and vsyscall vmas.
One can already use produced core dump with gdb, there is also
a quick simple demo https://asciinema.org/a/18936

 .

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>