RFR: 8298225: [AIX] Disable PPC64LE continuations on AIX [v3]

[Tyler Steele]

> This small change adds an import to the generated ad_ppc.cpp file to allow it to find Contiuations::enabled, and sets VMContinuations to false on AIX.

Tyler Steele has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into build/aix/continuation-enabled
 - Add continuation.hpp to adlc/main.cpp
 - Set VMContinuations to false on AIX
 - Restore 5 arg constructor for SystemProcess

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11550/files
  - new: https://git.openjdk.org/jdk/pull/11550/files/347a1091..5fb94181

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11550&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11550&range=01-02

  Stats: 9044 lines in 271 files changed: 6068 ins; 2119 del; 857 mod
  Patch: https://git.openjdk.org/jdk/pull/11550.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11550/head:pull/11550

PR: https://git.openjdk.org/jdk/pull/11550