RFR: 8261957: [PPC64] Support for Concurrent Thread-Stack Processing

[Martin Doerr]

I'd like to support Concurrent Thread-Stack Processing on PPC64. This will be needed by ShenandoahGC and zGC when implemented. Maybe for other purposes in the future, too.
I'm using conditional trap instructions by default, so we don't need the extra stubs unless -XX:-UseSIGTRAP is used.

Original change: https://github.com/openjdk/jdk/commit/b9873e18

-------------

Commit messages:
 - Update for JDK-8255233
 - Support large offsets in C1/C2 stubs. Add comments.
 - 8261957: [PPC64] Support for Concurrent Thread-Stack Processing

Changes: https://git.openjdk.java.net/jdk/pull/2841/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2841&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261957
  Stats: 227 lines in 16 files changed: 177 ins; 10 del; 40 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2841.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2841/head:pull/2841

PR: https://git.openjdk.java.net/jdk/pull/2841