RFR: 8303485: Replacing os.name for operating system customization

[Roger Riggs]

Improvements to support OS specific customization for JDK internal use:
 - To select values and code; allowing elimination of unused code and values
 - Optionally evaluated by build processes, compilation, or archiving (i.e. CDS)
 - Simple API to replace adhoc comparisons with `os.name`
 - Clear and consistent use across build, runtime, and JDK modules
 
The PR includes updates within java.base to use the new API.

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Commit messages:
 - 8303485: Replacing os.name for operating system customization

Changes: https://git.openjdk.org/jdk/pull/12931/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12931&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303485
  Stats: 435 lines in 13 files changed: 322 ins; 39 del; 74 mod
  Patch: https://git.openjdk.org/jdk/pull/12931.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12931/head:pull/12931

PR: https://git.openjdk.org/jdk/pull/12931