syscalls -- Add sys_sigaltstack

Will need it to retrieve signal stack parameters.
Note for simplicty I declared the syscall having
void * arguments, while libc has it as stack_t *.
This need to be cleaned up one day (because
we use similar structure for sigframe definition
and need to merge both definitions down).

ARM syscall def from alekskartashov@

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>