[jdk8u] RFR: 8304053: Revert os specific stubs for SystemMetrics [v4]

[Adam Farley]

> This change excludes the duplicated class SystemMetrics, and ensures we use the platform-specific version of the SystemMetrics class instead.

Adam Farley has updated the pull request incrementally with one additional commit since the last revision:

  Adding comment
  
  Signed-off-by: Adam Farley <adfarley at redhat.com>

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Changes:
  - all: https://git.openjdk.org/jdk8u/pull/40/files
  - new: https://git.openjdk.org/jdk8u/pull/40/files/b6a882fe..02ec9735

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk8u&pr=40&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk8u&pr=40&range=02-03

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u/pull/40.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u pull/40/head:pull/40

PR: https://git.openjdk.org/jdk8u/pull/40