RFR: 8301119: Support for GB18030-2022

[Naoto Sato]

Upgrading the GB18030 charset in the JDK to the latest 2022 standard. Since this is not a compatible upgrade to the existing mapping, a new system property `jdk.charset.GB18030` is introduced. If it is set to "2000", the mapping falls back to the existing mapping based on the 2000 standard, otherwise, it defaults to 2022 mapping. Refer to the corresponding CSR for more detail.

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Commit messages:
 - Check initPhase and don't call System.getProperty if in phase 1
 - Some clean-up
 - Some more fixes
 - removed unnecessary method name composition
 - Removed unnecessary imports
 - aliases fix
 - Move GB18030 into standard charsets provider
 - indentation
 - Removed unused annotation
 - Removed null check
 - ... and 4 more: https://git.openjdk.org/jdk/compare/574b48c6...0f3c25ce

Changes: https://git.openjdk.org/jdk/pull/12518/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12518&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301119
  Stats: 283 lines in 14 files changed: 129 ins; 41 del; 113 mod
  Patch: https://git.openjdk.org/jdk/pull/12518.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12518/head:pull/12518

PR: https://git.openjdk.org/jdk/pull/12518