[Solaris] Broken jdk8u build after JDK-8266391

OpenJDK8u doesn't have a copy of SystemMetrics (which the code references in Metrics.java) on Solaris, so the build fails with this error:

12:19:21 Compiling 9787 files for BUILD_JDK
12:20:01 /export/home/jenkins/sshagent/workspace/build-scripts/jobs/jdk8u/jdk8u-solaris-x64-temurin/workspace/build/src/jdk/src/share/classes/jdk/internal/platform/Metrics.java:57: error: cannot find symbol
12:20:01 return SystemMetrics.instance();
12:20:01 ^
12:20:01 symbol: variable SystemMetrics
12:20:01 location: interface Metrics

To resolve this issue, I suggest that we follow the approach taken in JDK-8303408.

I've already created the fix and tested it on Adoptium sparc and x64 solaris machines, so we know this fixes the issue. When it's the *right* fix is a bigger question.

Thoughts welcome. :)