[jdk8u-dev] RFR: 8303408 : [AIX] Broken jdk8u build after JDK-8266391

[Deepa Kumari]

The AIX build fails  after [JDK-8266391](https://bugs.openjdk.org/browse/JDK-8266391) was backported to jdk8. So, We added null returning SystemMetrics class an aix platform specific class that fixes the build error.

Reported Issue : [JDK-8303408](https://bugs.openjdk.org/browse/JDK-8303408)

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Commit messages:
 - 8303408 : [AIX] Broken jdk8u build after JDK-8266391

Changes: https://git.openjdk.org/jdk8u-dev/pull/274/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=274&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303408
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/274.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/274/head:pull/274

PR: https://git.openjdk.org/jdk8u-dev/pull/274