RFR: 8261957: [PPC64] Support for Concurrent Thread-Stack Processing [v2]

[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

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  cleanup MacroAssembler::safepoint_poll

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2841/files
  - new: https://git.openjdk.java.net/jdk/pull/2841/files/68326ddc..66a90b69

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2841&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2841&range=00-01

  Stats: 26 lines in 1 file changed: 7 ins; 10 del; 9 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