RFR: JDK-8260372: [PPC64] Add support for JDK-8210498 and JDK-8222841

[Niklas Radomski]

Introduces support for _nmethod entry barriers_ and _c2i entry barriers_ on the ppc platform. Those are required to enable concurrent class unloading for compatible garbage collectors, such as Shenandoah or zGC.

_This is a preparational change for the Shenandoah GC port to ppc. As such, it introduces features that the current version doesn't make use of, but that are required for the upcoming change. This way, the scope of the upcoming change is limited to Shenandoah-specific functionality; making its review a little easier._

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Commit messages:
 - Remove extraneous whitespace
 - [PPC64] Introduce nmethod_entry_barrier and c2i_entry_barrier

Changes: https://git.openjdk.java.net/jdk/pull/2432/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2432&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260372
  Stats: 264 lines in 10 files changed: 254 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2432.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2432/head:pull/2432

PR: https://git.openjdk.java.net/jdk/pull/2432