FROMLIST: softirq: Add generic accessor to percpu softirq_pending data

In a previous iteration of this patch series, I was checking:

   per_cpu(irq_stat, cpu).__softirq_pending

which resulted in build errors on s390.

This patch tries to create a generic accessor to this percpu
softirq_pending data.

This interface is inherently racy as its reading percpu data
without a lock. However, being able to peek at the softirq
pending data allows us to make better decisions about rt task
placement vs just ignoring it.

On s390 this call returns 0, which maybe isn't ideal but
results in no functional change from what we do now.

TODO: Heiko suggested changing s390 to use a proper per-cpu
irqstat variable instead.

Feedback or suggestions for better approach here would be
welcome!

Cc: John Dias <joaodias@google.com>
Cc: Connor O'Brien <connoro@google.com>
Cc: Rick Yiu <rickyiu@google.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Qais Yousef <qyousef@google.com>
Cc: Chris Redpath <chris.redpath@arm.com>
Cc: Abhijeet Dharmapurikar <adharmap@quicinc.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: kernel-team@android.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/lkml/20221116075929.453876-2-jstultz@google.com/
Change-Id: I333565755ef3804d1669e1c8328b2a5d72dd3a75
Bug: 168521633