RFR: 8276927: [PPC64] Port shenandoahgc to linux on ppc64le

[Niklas Radomski]

Port the Shenandoah garbage collector (JDK-8241457)[https://bugs.openjdk.java.net/browse/JDK-8241457] to linux on ppc64le.

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Commit messages:
 - Port shenandoahgc to linux on ppc64le

Changes: https://git.openjdk.java.net/jdk/pull/6325/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6325&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276927
  Stats: 1526 lines in 8 files changed: 1524 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6325.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6325/head:pull/6325

PR: https://git.openjdk.java.net/jdk/pull/6325