cr-restore: align the stack on 8-byte boundary

The section 5.2.1.2 of the AAPCS says that the stack must be 8-byte aligned
and this rule is broken when the thread restore_task_with_children()
is forked by the function fork_with_pid() since the variable ca
and its field stack are likely to be 4-byte aligned.

This patch forces 8-byte alingment of the field cr_clone_arg::stack.
This made the following tests pass on ARM:
* static/shm,
* static/ipc_namespace.

Particulary the unaligned stack results in incorrect passing
of the 64-bit argument to the function snprintf() in the function
sysctl_write_u64().

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>