x86: push old stack address on irqstack for unwinder

Impact: Fixes dumpstack and KDB on 64 bits

This re-adds the old stack pointer to the top of the irqstack to help
with unwinding.  It was removed in commit d99015b1abbad743aa049b439c1e1dede6d0fa49
as part of the save_args out-of-line work.

Both dumpstack and KDB require this information.

Signed-off-by: Martin Hicks <mort@sgi.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>