sched: disable IPIs while polling wakeup queue before idle

The scheduler polls the wakeup queue when idle for a short time before HLTing
in order to avoid the expensive HLT instruction if a wakeup arrives early.
This patch extends this to also disable remote wakeups during the polling
period, reducing the waking cpu's need to issue an IPI, whicj requires an
exit. This helps synchronous multithreaded workloads, where threads block
and wake each other.

Together with the following patch, netperf throughtput increases from ~17Gbps
to ~19Gbps, and the context switch benchmark improves from

$ run tests/tst-ctxsw.so
       345 colocated
      5761 apart
       633 nopin

to

$ run tests/tst-ctxsw.so
       347 colocated
       598 apart
       467 nopin