RFR: 8254696: safepointMechanism_aix needs adaptation for JDK-8253180

[Martin Doerr]

JDK-8253180 changed SafepointMechanism::default_initialize(). SafepointMechanism::pd_initialize() in
safepointMechanism_aix.cpp needs the same changes in order to fix build and initialization.

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Commit messages:
 - 8254696: safepointMechanism_aix needs adaptation for JDK-8253180

Changes: https://git.openjdk.java.net/jdk/pull/635/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=635&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254696
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/635.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/635/head:pull/635

PR: https://git.openjdk.java.net/jdk/pull/635