x86/restorer: add sigreturn to compat mode

Do pure 32-bit sigreturn.
Change code selector, do 0x80 rt_sigreturn.
We should have here remapped 32-bit vDSO, all should be fine.

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