[8u] RFR: Backport 8161993 G1 crashes if active_processor_count changes during startup

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8161993

JDK9 changeset: 
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/8986e5b85e73

webrev: http://cr.openjdk.java.net/~dholmes/8161993/webrev.8u/

The changes for 8147910 are there only for reference.

The backport was straight-forward but couldn't apply directly due to 
file/directory renaming. Also in 9 we had already changed 
os::processor_count() to os::active_processor_count() and then to 
os::initial_active_processor_count(), but in 8u we skip the middle step.

Thanks,
David