Rework of the X86 irq stack handling:
The irq stack switching was moved out of the ASM entry code in course of
the entry code consolidation. It ended up being suboptimal in various
ways.
- Make the stack switching inline so the stackpointer manipulation is not
longer at an easy to find place. This also gets rid of the indirect
call.
- Avoid the double stack switch in interrupt return and reuse the
interrupt stack for softirq handling