x86/compat: Use do_full_int80() instead of jump to ia32 mode

Kernel will send signal with ABI of *syscall* that made rt_sigacation().
In sigaction_compat_abi():
>        if (in_ia32_syscall())
>                   act->sa.sa_flags |= SA_IA32_ABI;

So, we can omit switching to ia32 mode and just call int80 like in other places.
We still need 32-bit argument for syscall (sig action pointer).

As call32_from_64() switches mode, it's .text should be placed in lower 4Gb,
which was the reason of segfault if linker places criu code above 4Gb.
Oops, my bad.

Fixes: #567

Reported-by: Andrey Vagin <avagin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>