RFR: JDK-8307520: set minimum supported CPU architecture to Power8 on AIX

[JoKern65]

In the compile flags, set the minimum supported CPU architecture to Power8 on AIX, because we do not want to support pwr7 architecture any more.

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Commit messages:
 - accidentially changed mode bits
 - JDK-8307520

Changes: https://git.openjdk.org/jdk/pull/13933/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13933&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307520
  Stats: 4 lines in 2 files changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13933.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13933/head:pull/13933

PR: https://git.openjdk.org/jdk/pull/13933