x86/crtools: rework arch_task_compatible

Detect task's personality by the size of register set, returned
by ptrace call with PTRACE_GETREGSET (more reliable).
The same approach uses strace from 4.8 version:
https://sourceforge.net/projects/strace/files/strace/4.8/



Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>