um: Enforce the usage of asm-generic/softirq_stack.h

The recent rework of the X86 irq stack switching mechanism broke UM as UM
pulls in the X86 specific variant of softirq_stack.h.

Enforce the usage of the asm-generic variant.

Fixes: 72f40a28 ("x86/softirq/64: Inline do_softirq_own_stack()")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Weinberger <richard@nod.at>