RFR (S) 8247310: Shenandoah: pacer should not affect interrupt status

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8247310

This was originally found by Aditya Mandaleeka when running sh/jdk11 tests, kudos goes to him for
analysing it. See the details in the bug report.

Fix:
  https://cr.openjdk.java.net/~shade/8247310/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey