i386: Execute stack overflow warning on interrupt stack v2

Previously the reporting printk would run on the process stack, which risks
overflow an already low stack. Instead execute it on the interrupt stack.
This makes it more likely for the printk to make it actually out.

It adds one not taken test/branch more to the interrupt path when
stack overflow checking is enabled. We could avoid that by duplicating
more code, but that seemed not worth it.

Based on an observation by Eric Sandeen.

v2: Fix warnings in some configs

Signed-off-by: Andi Kleen <andi@firstfloor.org>
Cc: mingo@elte.hu
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>