RFR: 8265989: System property for the native character encoding name

[Naoto Sato]

After some internal discussion, we thought it was good to expose the native environment's default character encoding, which Charset.defaultCharset() is currently based on. This way applications will have a better migration path after the [JEP 400](https://openjdk.java.net/jeps/400) is implemented, in which Charset.defaultCharset() will return UTF-8, but the value of this new system property will remain intact. A [CSR](https://bugs.openjdk.java.net/browse/JDK-8266075) has been filed with more detailed information.

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Commit messages:
 - 8265989: System property for the native character encoding name

Changes: https://git.openjdk.java.net/jdk/pull/3777/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3777&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265989
  Stats: 15 lines in 3 files changed: 8 ins; 3 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3777.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3777/head:pull/3777

PR: https://git.openjdk.java.net/jdk/pull/3777