[8u] Request for approval for CR 8147910 and CR 8161993

[David Holmes]

Please approve these two related backports for 8u. They complement the 
existing backport of:

JDK-6515172: Runtime.availableProcessors() ignores Linux taskset command

which reports actual number of active processors when running in a 
cpuset constrained environment (like Docker). That by itself won't cause 
G1 to adjust the number of GC worker threads, and the other GC's may be 
adversely impacted if the number of processors were to change during 
execution of the VM. To address that we also need to back port the 
following:

- JDK-8147910 Cache initial active_processor_count

Bug: https://bugs.openjdk.java.net/browse/JDK-8147910
JDK9 changeset: 
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/42bdfbb535a2

This changes applies cleanly other than a unified logging line which was 
deleted.

- JDK-8161993 G1 crashes if active_processor_count changes during startup

Bug: https://bugs.openjdk.java.net/browse/JDK-8161993
JDK9 changeset: 
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/8986e5b85e73

This change could not apply directly due to file and directory renaming 
in JDK 9. For good measure I had it re-reviewed:

http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2016-December/019361.html

Thank you,
David