x86: don't rely on __softirq_pending to be the first field in irq_cpustat_t

x86: don't rely on __softirq_pending to be the first field in irq_cpustat_t

This is even more so as the field doesn't have a comment to that effect
in the structure definition.

Once modifying the respective assembly code, also convert the
IRQSTAT_shift users to do a 32-bit shift only (as we won't support 48M
CPUs any time soon) and use "cmpl" instead of "testl" when checking the
field (both reducing code size).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>