RFR: 8303485: Replacing os.name for operating system customization [v7]

[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.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  clarify Linux

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12931/files
  - new: https://git.openjdk.org/jdk/pull/12931/files/8372978f..bfc3de93

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12931&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12931&range=05-06

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12931.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12931/head:pull/12931

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