[jdk17u-dev] RFR: 8277846: Implement fast-path for ASCII-compatible CharsetEncoders on ppc64

[Martin Doerr]

Clean backport of JDK-8277846. x86 version is already in 17.0.2 (JDK-8274242).

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Commit messages:
 - Backport a5f2a58ba4ac25f4bd66f1f1f4c036a4f0024229

Changes: https://git.openjdk.java.net/jdk17u-dev/pull/81/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u-dev&pr=81&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277846
  Stats: 78 lines in 4 files changed: 46 ins; 14 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk17u-dev/pull/81.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u-dev pull/81/head:pull/81

PR: https://git.openjdk.java.net/jdk17u-dev/pull/81