x86/entry/64: Move do_softirq_own_stack() to C

The first step to get rid of the ENTER/LEAVE_IRQ_STACK ASM macro maze.  Use
the new C code helpers to move do_softirq_own_stack() out of ASM code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200521202117.870911120@linutronix.de